Onion casserole turns three pounds of sweet onions into a soft, jammy, cheesy side dish, layered with cheddar, smoky Gouda and Parmesan, dotted with butter and baked until it bubbles and browns in spots. It’s a striking holiday or Sunday side next to roasted meat, and it needs no stovetop work at all.
How to make onion casserole: toss 1/4-inch sweet onion rings with salt, dry mustard, garlic powder, smoked paprika and cayenne, layer them in a 9×13 dish with cheddar, Gouda and Parmesan, dot with butter, and bake at 350°F covered for about 45 minutes, then uncovered for about 30 minutes more.
Why You’ll Love It
- No skillet. The onions go into the dish raw and soften in the oven.
- Three cheeses: mild cheddar, smoky Gouda and a Parmesan top.
- Feeds 10 from one 9×13 dish.
- 15 minutes of prep; the oven handles the rest.
Ingredient Notes
Sweet onions. Three pounds of Vidalia or other sweet onions, about 4 large, cut crosswise into 1/4-inch-thick slices and separated into rings.
Spices. Kosher salt, dry mustard powder, garlic powder and smoked paprika, plus a pinch of cayenne if you like a little heat.
Cheeses. Four ounces each of shredded mild cheddar and smoked Gouda (or more cheddar in place of the Gouda), split between two layers, and about 1/3 cup of grated Parmesan for the top.
Butter. Half a stick of unsalted butter, diced and scattered over the top before baking.
Ingredients
- Cooking spray, for the dish
- 3 pounds Vidalia or sweet onions, about 4 large, cut crosswise into 1/4-inch-thick slices
- 1 1/2 teaspoons kosher salt
- 1 teaspoon dry mustard powder
- 1 teaspoon garlic powder
- 1 teaspoon smoked paprika
- 1/4 teaspoon cayenne pepper, optional
- 4 ounces mild cheddar cheese, shredded, about 1 cup, divided
- 4 ounces smoked Gouda or cheddar cheese, shredded, about 1 cup, divided
- 1 1/4 ounces grated Parmesan cheese, about 1/3 cup store-bought grated
- 4 tablespoons unsalted butter, 1/2 stick, diced
- 2 tablespoons fresh parsley, finely chopped, for garnish, optional
How to Make Onion Casserole
1. Heat the oven and season the onions. Heat the oven to 350°F. Lightly coat a 9×13-inch baking dish with cooking spray. Separate the sliced onions into rings and put them in a large bowl. Add the salt, dry mustard, garlic powder, smoked paprika and cayenne, if using, and toss to coat.
2. Layer half the onions. Arrange half of the onion rings in an even layer in the dish. Sprinkle evenly with half of the cheddar and half of the Gouda (about 1/2 cup each).
3. Add the rest and the butter. Arrange the remaining onions on top and sprinkle with the remaining cheddar and Gouda. Sprinkle evenly with the Parmesan. Scatter the diced butter on top.
4. Bake covered. Cover the dish with aluminum foil. Bake about 45 minutes, until the onions are softened and the casserole is bubbling.
5. Bake uncovered. Uncover and bake about 30 minutes more, until the cheese on top is melted and browned in spots and the center reaches 165°F. Sprinkle with parsley if you like and serve.
Which Onions to Use and How They Soften in the Oven
You might expect to soften the onions in a skillet first. Not here. The raw rings go straight into the dish and the oven does the softening, in two stages.
Choose sweet onions. Vidalia or other sweet onions are the onion of choice for this dish. You need about 3 pounds, roughly 4 large.
Slice them thin and separate the rings. Cut the onions crosswise into 1/4-inch slices, then pull them apart into rings.
Cover first. With foil on, the first 45 minutes soften the onions until they’re tender and the casserole bubbles.
Then uncover. The last 30 minutes without foil melt the cheese on top and brown it in spots.
| Stage | Time at 350°F | What you’re looking for |
|---|---|---|
| Covered with foil | About 45 minutes | Onions soft, casserole bubbling |
| Uncovered | About 30 minutes | Cheese melted and browned in spots |
Prefer the skillet route? Cook the onions slowly in butter with salt and pepper over low heat for 45 to 60 minutes, stirring every few minutes, until they’re soft, translucent and just slightly golden. They should barely sizzle, not brown, and they’ll release liquid and cook down a lot. Then mix them into a creamy cheese base and bake (see Variations).
Tips for Success
Why are my onions still crunchy?
The covered stage was too short. Keep the foil on until the onions are soft and the casserole is bubbling, about 45 minutes, before you uncover it.
Why is the top pale?
It needs the uncovered time. Bake about 30 minutes without foil, until the cheese is melted and browned in spots.
Variations
- Creamy with a cracker top: mix 2 pounds of slowly sautéed sweet onions with 1/4 cup mayonnaise, 1/4 cup sour cream, 1/4 cup grated Parmesan and 2 ounces grated cheddar, top with 15 crushed Ritz crackers and bake at 375°F for 30 minutes until bubbly and golden.
- All cheddar: use cheddar in place of the smoked Gouda.
- No heat: leave out the cayenne.
Storage & Reheating
Onion casserole is best fresh from the oven. Refrigerate leftovers within 2 hours in shallow, covered containers and eat them within 3 to 4 days. Reheat covered until it reaches 165°F; USDA FSIS does not recommend reheating leftovers in a slow cooker.
What to Serve With It
Serve it as a side for a roast or chops: it’s a natural fit with slow cooker 4-ingredient onion beef brisket, slow cooker Amish onion gravy pork roast or French onion pork chops. For a chicken dinner, pair it with 4-ingredient French onion chicken. If you’re building a casserole spread, add 4-ingredient slow cooker potato casserole or Ritz cracker chicken casserole.
Frequently Asked Questions
Which onions should I use?
Vidalia or other sweet onions, about 3 pounds. Cut them into 1/4-inch slices and separate the rings.
How do you make onions in the oven?
Layer the seasoned raw rings in a dish with cheese and butter, cover with foil and bake at 350°F about 45 minutes until soft, then uncover for about 30 minutes to brown the top.
Can I make it creamy with a cracker topping?
Yes. Sauté the onions first, stir them into mayonnaise, sour cream, Parmesan and cheddar, top with crushed Ritz crackers and bake at 375°F for 30 minutes.
Final Notes
Slice all the onions before you start layering; it goes much faster once everything is ready in one bowl. Tell me in the comments what you served it with.
Onion Casserole
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Ingredients
- cooking spray for the dish
- 3 pounds Vidalia or sweet onions about 4 large, cut crosswise into 1/4-inch-thick slices
- 1 1/2 teaspoons kosher salt
- 1 teaspoon dry mustard powder
- 1 teaspoon garlic powder
- 1 teaspoon smoked paprika
- 1/4 teaspoon cayenne pepper optional
- 4 ounces mild cheddar cheese shredded, about 1 cup, divided
- 4 ounces smoked Gouda or cheddar cheese shredded, about 1 cup, divided
- 1 1/4 ounces grated Parmesan cheese about 1/3 cup store-bought grated
- 4 tablespoons unsalted butter 1/2 stick, diced
- 2 tablespoons fresh parsley finely chopped, for garnish, optional
Instructions
- Heat the oven to 350°F. Lightly coat a 9x13-inch baking dish with cooking spray. Separate the sliced onions into rings and put them in a large bowl. Add the salt, dry mustard, garlic powder, smoked paprika and cayenne, if using, and toss to coat.
- Arrange half of the onion rings in an even layer in the dish. Sprinkle evenly with half of the cheddar and half of the Gouda (about 1/2 cup each).
- Arrange the remaining onions on top and sprinkle with the remaining cheddar and Gouda. Sprinkle evenly with the Parmesan. Scatter the diced butter on top.
- Cover the dish with aluminum foil. Bake about 45 minutes, until the onions are softened and the casserole is bubbling.
- Uncover and bake about 30 minutes more, until the cheese on top is melted and browned in spots and the center reaches 165°F. Sprinkle with parsley if you like and serve.
Notes
Nutrition
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