Old-Fashioned Scalloped Potatoes Recipe

2 1/2 pounds baking potatoes, peeled
2 tablespoons butter
2 tablespoons all-purpose flour
1 cup half and half or heavy cream
1 cup whole milk
4 ounces processed cheese, such as Velveeta, cubed
1/2 teaspoon salt, or to taste
1/4 teaspoon black pepper, or to taste
1 cup shredded Cheddar
1 medium onion, finely chopped
1/4 cup grated Parmesan

1. Preheat the oven to 350 degrees F. Coat a 9-inch-square baking pan with nonstick cooking spray.
2. Cut potatoes into thin slices. Place in a bowl of cold water. Melt butter in a saucepan over medium heat. Stir in flour and cook 1 minute. Add half and half and milk slowly, stirring constantly. Reduce heat to low, stir in processed cheese, salt and pepper. Cook until cheese melts, stirring constantly. Remove from heat.
3. Drain potatoes well and arrange one third of them in the pan. Season with salt and pepper. Top with one-third of the onions and one-third of the Cheddar. Pour a third of the cheese sauce over all. Repeat layers twice.
4. Coat a sheet of aluminum foil with nonstick cooking spray. Tent the foil with the sprayed side down over the casserole. Place on a foil-lined oven rack. Bake for 1½ hours.
5. Remove foil, sprinkle with Parmesan, and bake for 20 to 30 minutes longer, until potatoes are tender. Serves 8.