Preheat oven to 350℉. Grease a 9x13-inch baking dish and set aside.
Melt butter in a large sauté pan over medium high heat. Add flour and whisk until a thick paste forms.
3 Tablespoons butter, 3 Tablespoons flour
Slowly add milk and then heavy cream, whisking until thick and creamy.
2 cups whole milk, 1 cup heavy cream
Add in cheese, seasoned salt, and pepper. Mix until combined. This is your bechamel sauce.
2 cups sharp cheddar cheese, 1 teaspoon seasoned salt, ½ teaspoon pepper
Add half the potatoes and onions to the baking dish and sprinkle with salt and pepper. Cover with half of the bechamel sauce.
salt, ½ teaspoon pepper, 6 medium-sized Yukon gold potatoes (or yellow potatoes), 1 small onion
Add remaining potatoes and onions with another sprinkle of salt and pepper, then pour on the remaining sauce. Cover with foil and bake for 1 hour.
Uncover, and add a layer of cheese to the top. Bake for 10 minutes until melted and a little browned, then remove, and serve!