Cook the Onions: In a small skillet, melt butter over medium heat. Add the onions and sauté until they soften slightly, about 4 to 5 minutes.

Make the Cheese Sauce: Meanwhile, in a medium bowl, mix together 1 cup of cheddar cheese, cream of chicken soup, 1 cup of heavy cream, garlic powder, thyme, 1/4 teaspoon salt, and black pepper until well combined.

Layer the Ingredients: Grease a 6-quart slow cooker with non-stick spray. Place half of the sliced potatoes in the bottom of the slow cooker, and season them with 1/4 teaspoon salt. Top with all of the cooked onions, spreading them evenly. Spoon half of the cheese sauce over the onions and potatoes.

Add Remaining Layers: Add the remaining potato slices on top, season with another 1/4 teaspoon salt, and pour the rest of the cheese sauce over the potatoes. Drizzle 1/2 cup of heavy cream evenly over the top.

Cook: Lay a double layer of paper towels over the top of the slow cooker, being careful not to let them touch the potatoes. Cover with the lid to help absorb moisture, which prevents the potatoes from becoming watery. Cook on high for 4-5 hours or on low for 7-8 hours.

Add the Cheese: After about 2 1/2 hours of cooking, sprinkle the remaining cheese over the potatoes and continue cooking until the potatoes are tender and the cheese is melted.