Cook macaroni in a large amount of boiling salted water just until tender. Drain. While macaroni cooks, melt butter in a large saucepan. Remove from heat. Blend in flour, mustard, salt, Worcestershire and pepper. Stir in milk. Cook over low heat, stirring constantly until sauce thickens and boils 1 minute. Remove from heat. Stir in cooked macaroni, tuna, and onion. Cut 1/2″ thick slice from the middle of tomato, save to decorate top. Thinly slice rest of tomato. Place half of the macaroni mixture in an oiled 2-quart baking dish. Top with the thin tomato slices and 4 slices cheese. Spoon remaining macaroni mixture over cheese layer. Cut 4 remaining cheese slices into 4 strips each (to make 16 strips). Arrange cheese strips, checkerboard fashion, or in any decorative pattern, on top of casserole. Cut reserved thick tomato slice into 6 wedges. Place on top of casserole. Bake for 30 minutes at 350-degree oven until bubbly hot. OR refrigerate until just before serving time. Bake 45 to 50 minutes.
Yield: 6 to 8 servings.