Melt 3 tablespoons butter; add 2 tablespoons flour. Blend in 3 cups of milk and 2 1/2 teaspoons of salt. Cook until smooth and thick. Add 6 cups of thinly sliced potatoes. Cover and simmer 7 minutes. Pour into a casserole or 8 x 12 inch pan. Bake 45 minutes at 375 degrees. This method keeps the scalloped potatoes from curdling.