Butter a 2-quart casserole. Cook macaroni in lightly salted water according to package directions; drain.

In a stainless steel, glass or enamel saucepan, sauté onion in butter until transparent. Stir in flour. Cook for 1 minute. Add milk.

Cook, stirring until sauce thickens. Add salt, Worcestershire sauce and cheese. Heat, stirring until cheese melts. Empty sour cream into a large bowl. Gradually add sauce, stirring constantly. Fold in cooked macaroni. Pour into casserole and sprinkle with bacon, if desired.

Bake at 375°F for 25 to 30 minutes or until bubbly.

Makes 6 servings.

Variation: Substitute 1/4 cup buttered bread crumbs, or 1/2 cup grated Cheddar cheese, for crumbled bacon, if desired.