Step 1

Spray a casserole dish or 9x13 pan with nonstick cooking spray and set aside. Preheat oven to 425° F.

Step 2

Peel potatoes and rinse. Slice into 1/4-inch pieces and set aside.

Step 3

In a deep skillet, melt butter over medium heat. Saute onions until lightly browned and fragrant, about 5 minutes.

Step 4

Sprinkle in flour and whisk until a smooth paste is created. Slowly whisk in milk and bring to simmer.

Step 5

Add in salt, black pepper, and nutmeg while waiting for the sauce to bubble. The sauce should thicken once it starts bubbling.

Step 6

Remove from heat and stir in heavy cream. Taste and adjust seasonings. Once you get the sauce to taste how you like, add in a little extra salt.

Step 7

Spread about 1/3 cup cream sauce into the casserole dish. Add about half the potatoes, being sure to separate from each other as best as possible.

Step 8

Season potatoes with salt and pepper. Top with half the remaining cream sauce and half the cheddar cheese.

Step 9

Top with remaining potatoes, sauce and cheese.

Step 10

Bake for 1 hour uncovered. Check after 30-40 minutes to ensure the top isn't getting too brown. If it is, cover with foil for remaining baking time.

Step 11

Let rest for 10-15 minutes before serving. Garnish with fresh herbs like chopped chives or parsley.