Use it to make mac and cheese! Pour the leftover sauce into a pot and add some additional grated cheese. Cook over low heat until the cheese is melted then stir in some cooked pasta. Eat it as is or spread it into a baking dish, top with bread crumb tossed with melted butter and bake until bubbly. You can also use leftover Béchamel Sauce to make a classic Mornay Sauce which is little more than adding more cheese. Pour this decadent cheesy sauce over vegetables or fish. Or, use leftover Béchamel Sauce to make a Croque Madame, a delicious sandwich stuffed ham, cheese, and béchamel sauce, grilled until golden, then topped with a fried egg.