Sauté onion in butter in a heavy skillet until tender. Add cubed pork, salt, and pepper; cook over medium heat 10 minutes or until pork is browned.
Cook macaroni according to package directions; drain. Combine tenderloin mixture, tomatoes, and macaroni; spoon into a 2-quart casserole. Combine cheese and sausage; sprinkle over top of casserole. Bake, uncovered, at 350° for 8 minutes or until cheese melts. Serve immediately.