Peel and thinly slice potatoes.
Dice 3-4 tablespoons onion.
Make white sauce by melting butter in saucepan over medium heat. Add flour and stir until well incorporated - about 2 minutes. Slowly pour in milk, increase heat to medium high and cook, stirring continuously, until sauce is thickened.
In greased 2 quart casserole dish, layer half of the potatoes, half of the onions and half of the seasonings. Pour half of the white sauce over the potatoes and repeat with the rest of the ingredients.
Cover and bake at 350 degrees for 45 minutes to one hour.
Remove lid and bake until potatoes are tender and the top is starting to brown - about 15 minutes.