Cook beef in a nonstick skillet over moderately high heat, breaking up meat with a spoon, until no longer pink. Transfer beef with a slotted spoon to a bowl, leaving fat in skillet. Add onion to skillet and cook, stirring, until softened, about 3 minutes. Stir in beef, tomatoes with puree, and Worcestershire sauce; simmer, stirring occasionally, 10 minutes. Salt and pepper to taste.

Melt butter in a large saucepan over moderate heat. Remove pan from heat and transfer 1 tablespoon of the butter to a medium bowl, leaving remaining butter in pan. Add bread crumbs to bowl with butter and stir to combine. Whisk flour into saucepan and cook over moderate heat, whisking, until mixture is pale brown, about 3 minutes. Whisk in milk in a slow, steady stream and simmer, whisking occasionally, until slightly thickened, about 6 minutes. Remove pan from heat and whisk until melted and mixture is almost smooth. Salt and pepper to taste. Keep sauce warm, covered, off heat.

Preheat oven to 400 degree F and lightly butter 6 ovenproof ramekins (about 2-cup capacity) or glass dishes.

Cook pasta in a 6-quart pot of boiling salted water, stirring occasionally, until al dente; drain in colander. Return pasta to pot and pour in cheese sauce. Gently stir pasta and sauce together until combined well.

Spoon about 1/2 cup meat mixture in bottom of each prepared ramekin and top evenly with macaroni and cheese. Sprinkle with bread crumb mixture and bake casseroles on a sheet pan in middle of oven until bubbling and golden brown, about 20 minutes. Makes 6 servings.