Preheat oven to 325°F. Cook macaroni. Run under cold water to cool. While that cooks, cut up bacon & brown. When halfway cooked, add onions. Drain on paper towels.
Take a large baking dish and put a layer of macaroni. Put slices of Velveeta on top of macaroni (try not to overlap). Sprinkle bacon & onion mixture over macaroni. Spoon on crushed tomato. Keep layering till macaroni is gone.