Preheat oven to 350 degrees.
Wash potatoes and remove eyes.Cut poatatoes into thin slices to measure about 4 cups. (I use my food processor)to slice the potatoes.Melt 3 Tbsp. butter-margine in a sauce panover low heat. Blend in flour and seasonings. Cook over low heat,stirring until mixture is smooth and bubbly. Remove from heat. Stir in carnation evaporated milk. Heat to boiling,stirring constantly. Boil and stir one minute.
In a greased 2qt. casserole, arrange potatoes in two layers,topping each with half the onion and 1/3 of the white sauce.Top with remaining potatoes and sauce.Dot with 1 tbsp. butter or margine. Cover bake 30 minutes. Uncover;bake 60 to 70 minutes oe until potatoes are tender. Let stand 5 to 10 minutes before serving.