Melt 3 Tbs. butter or margarine. Add 2 Tbs flour & 2 1/2 tsp. salt, blend very gradually, add 3 2/3 cups milk (water with evaporated milk makes it especially rich looking & creamy). Cook until smooth & thick, stirring constantly. Add 6 cups thickly sliced potatoes. Cover. Simmer 7 minutes. Pour into shallow baking dish. Bake, uncovered, in 350 degree oven until tender, about 35 minutes. They will have a good brown crust.