Mashed Potato Soup turns leftover mashed potatoes into a thick, creamy cheddar soup in 30 minutes. Four cups of mashed potatoes go into a butter and flour roux with whole milk and vegetable broth, and a cup of shredded cheddar melts in at the end. There is nothing to peel, chop or boil, and the pot serves 8.
How to make it: Melt the butter, whisk in the flour, then whisk in the milk and let it thicken for about 5 minutes. Add the broth and bring it to a simmer, stir in the mashed potatoes, cheddar, salt and pepper, and keep stirring until the cheese melts and the soup is hot all the way through.
Why You’ll Love It
- Leftovers become dinner: the extra mashed potatoes from a holiday meal turn into a whole new pot of soup.
- Quick: the potatoes are already cooked, so the soup only has to thicken and heat through.
- One pot: the roux, milk, broth, potatoes and cheese all go into the same large pot.
- Top it your way: set out sour cream, extra cheddar, bacon and green onions and let everyone load a bowl.
Ingredient Notes
Leftover mashed potatoes: Four cups, straight from the refrigerator. How much you need, how old they can be and how their texture shapes the soup are all covered below the steps.
Butter and flour: Four tablespoons of butter and 1/4 cup of all-purpose flour make the roux that helps thicken the soup.
Whole milk: Two cups. 2% milk also gives the soup a creamy texture.
Broth: Four cups of vegetable broth. Chicken broth works in its place, and low-sodium chicken broth is fine too.
Cheddar: One cup of shredded medium cheddar melts right into the soup.
Salt and pepper: Half a teaspoon each of kosher salt and black pepper to start.
Ingredients
- 4 tablespoons butter
- 1/4 cup all-purpose flour
- 2 cups whole milk
- 4 cups vegetable broth, or chicken broth
- 4 cups leftover mashed potatoes
- 1 cup medium cheddar cheese, shredded
- 1/2 teaspoon kosher salt, or more to taste
- 1/2 teaspoon black pepper, or more to taste
- Sour cream, extra shredded cheddar cheese, chopped cooked bacon and sliced green onions, for serving (optional)
How to Make Mashed Potato Soup
1. Make the roux. Measure the flour and milk before you start. In a large pot over medium-low heat, melt the butter. Slowly sprinkle in the flour, whisking constantly, until it forms a thick paste, less than 30 seconds. Do not let it burn.
2. Add the milk. Whisk in the milk. Let it simmer for about 5 minutes, whisking frequently, until it thickens.
3. Add the broth. Add the broth and bring the soup to a simmer, with a low boil at the edges of the pot. Turn the heat up to medium if needed, and whisk frequently so the milk does not burn.
4. Stir in the potatoes and cheese. Add the mashed potatoes, cheddar, salt and pepper. Stir until the potatoes break up and the cheese melts.
5. Heat through and serve. The soup is ready when it has thickened, the cheese has fully melted and it is hot all the way through (165°F). Taste, add more salt and pepper if needed, and serve with sour cream, extra cheddar, bacon and green onions.
Turning Leftover Mashed Potatoes into Soup: How Much, How Old, How Thick
How much to use. A pot of this soup needs at least 3 cups of leftover mashed potatoes. The recipe above uses 4 cups with 2 cups of milk and 4 cups of broth.
How old they can be. Use mashed potatoes that went into the refrigerator within 2 hours of cooking and are no more than 3 to 4 days old, the USDA FSIS limit for leftovers. Potatoes that sat out at room temperature for more than 2 hours (1 hour on a day above 90°F) should be thrown away, not turned into soup.
Smooth or chunky. The soup takes on the texture of your potatoes. Smooth mashed potatoes make a silky soup; chunky ones make a chunkier soup. Keep stirring after they go in until they break up. For a completely smooth bowl, run an immersion blender through the pot.
Too thick or too thin. If the soup gets thicker than you like, stir in a splash of broth or milk. If it is too thin, let it cook a little longer until it reaches the thickness you want, or stir in more mashed potatoes.
Tips for Success
How do I keep the milk from scorching?
Have the flour and milk measured before the butter melts, since the roux comes together in less than 30 seconds. Keep the heat at medium-low for the butter, flour and milk. Once the broth is in, raise the heat to medium only if the pot will not simmer, and whisk often so the milk does not burn on the bottom.
Should all the cheese go into the pot?
It can. For a cheesier top, stir 3/4 cup into the soup and save the rest to sprinkle over the bowls.
When should I add more salt?
At the end. Start with the 1/2 teaspoon each of salt and pepper, taste the soup once the potatoes and cheese are in, and add more only if it needs it.
Variations
- Golden onion: before the flour goes in, cook 1/2 small onion, finely minced, in the melted butter over medium heat, stirring now and then, until golden brown, about 10 minutes. Then add the flour and carry on.
- Chicken broth: use 4 cups of chicken broth, regular or low-sodium, in place of the vegetable broth.
- All milk, no broth: for a smaller pot, melt 2 tablespoons of butter, whisk in 1 tablespoon of flour, add 2 1/2 cups of 2% milk and 3 cups of mashed potatoes, and cook on medium-low for 10 to 15 minutes, until smooth.
- No leftovers: peel and dice 2 large russet potatoes, boil them in salted water until fork tender and mash them with 2 tablespoons of butter. With that amount, use 2 cups of chicken broth and 1 cup of milk.
- Loaded with vegetables: soften 1/2 cup of chopped shallots and 1/2 cup of finely diced celery in butter with 1 tablespoon of bacon drippings or olive oil. Add 2 teaspoons of chopped garlic and 2 tablespoons of dried chives with the liquid, and take the pot off the heat to stir in 1/2 cup of sour cream and 1/2 teaspoon of paprika with the cheddar.
Storage & Reheating
USDA FSIS says to refrigerate leftovers within 2 hours. Divide the soup into smaller, shallow covered containers so it cools quickly, and eat it within 3 to 4 days. It can be frozen for 3 to 4 months, but FSIS lists cream sauces among foods that don’t freeze well and notes that cream can separate and look curdled, so expect the texture of this milk and cheese soup to change. Thaw frozen soup in the refrigerator. To reheat, bring the soup to a rolling boil in a saucepan on the stove, stirring so the bottom doesn’t scorch, and add a splash of broth or milk if it has thickened. Leftovers should reach 165°F. Do not reheat it in a slow cooker.
What to Serve With It
Top each bowl with sour cream, shredded cheddar, crumbled bacon and sliced green onions, or keep it simple with chives, dried parsley and cracked black pepper. For more ways to use up leftover mashed potatoes, bake cheesy mashed potato puffs or garlic Parmesan mashed potato cups. When you want a potato soup made from scratch, try baked potato soup, creamy baked potato soup, 5-ingredient slow cooker loaded potato soup or crockpot potato broccoli cheddar soup.
Frequently Asked Questions
What gives potato soup more flavor?
Start in the pot: a finely minced onion cooked in the butter until golden before the flour goes in, or shallots, celery and garlic softened with a tablespoon of bacon drippings. Then finish each bowl with cheddar, bacon, sour cream, green onions, chives or cracked black pepper.
What are the four ingredients in potato soup?
In this soup, the four main ones are leftover mashed potatoes, milk, broth and cheddar cheese. Butter and flour make the roux that helps thicken it, and salt and pepper season it.
Can I make mashed potato soup without milk?
Yes. Use vegetable or chicken broth in place of the milk. For a dairy-free soup, use plant-based butter and milk; the taste will change. For a vegan pot, pair those with vegetable broth.
Can I use mashed sweet potatoes?
Yes. Leftover mashed sweet potatoes turn it into a sweet potato soup, made the same way.
Can I make it gluten-free?
Yes. Use a gluten-free flour for the roux.
Final Notes
Start with mashed potatoes that are no more than 3 to 4 days old, keep the heat low while the milk goes in, and set the thickness with a splash of broth or a few more potatoes. If you make this soup, tell me how it turned out in the comments.
Mashed Potato Soup
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Ingredients
- 4 tablespoons butter
- 1/4 cup all-purpose flour
- 2 cups whole milk
- 4 cups vegetable broth or chicken broth
- 4 cups mashed potatoes leftover
- 1 cup cheddar cheese medium, shredded
- 1/2 teaspoon kosher salt or more to taste
- 1/2 teaspoon black pepper or more to taste
- sour cream, cheddar cheese, bacon and green onions optional, for serving: extra shredded cheddar, chopped cooked bacon, sliced green onions
Instructions
- Measure the flour and milk before you start. In a large pot over medium-low heat, melt the butter. Slowly sprinkle in the flour, whisking constantly, until it forms a thick paste, less than 30 seconds. Do not let it burn.
- Whisk in the milk. Let it simmer for about 5 minutes, whisking frequently, until it thickens.
- Add the broth and bring the soup to a simmer, with a low boil at the edges of the pot. Turn the heat up to medium if needed, and whisk frequently so the milk does not burn.
- Add the mashed potatoes, cheddar, salt and pepper. Stir until the potatoes break up and the cheese melts.
- The soup is ready when it has thickened, the cheese has fully melted and it is hot all the way through (165°F). Taste, add more salt and pepper if needed, and serve with sour cream, extra cheddar, bacon and green onions.
