My Mother-in-Law's Cheese Sauce...ssshh Don't Tell Her

In a medium saucepan, bring butter, milk, minced garlic, and minced onion to a low boil, stirring frequently. Dissolve cornstarch in 1 tablespoon water and whisk into milk mixture. Continue stirring vigorously until desired thickness.
When milk mixture is thick enough, tear cheese slices into pieces and slowly stir them into the sauce, melting gradually, until desired cheesiness. Serve over your favorite pasta, vegetable, or side dish.