In a saucepan, melt butter; mix in flour and seasonings. Gradually add the milk. Stir constantly over low heat until thickened.
Using a greased 1 1/2-quart casserole, arrange potatoes, tuna, and onion, and top with sauce. Bake for about 50 to 60 minutes at 350 F in a preheated oven.
Remove from the oven and serve immediately. Enjoy!