Preheat oven to 400 °F and butter casserole dish

Soak potatoes in water until sauce is made

To make sauce, melt butter over medium heat. Add in flour and stir continuously for about a minute, until it begins to smell nutty.

Pour milk into roux, stirring periodically, until it has thickened

Add salt, pepper, and garlic powder

Remove from heat, add cheese and stir until melted

Line casserole dish with half of potatoes, then half of sliced onions. Top with half of sauce. Add remaining potatoes, onion, and top with sauce. Cover with foil or lid.

Bake 1 hour in the oven