1. Preheat oven to 450 degrees F. Cook spaghetti according to package directions and drain. Brown bacon in large skillet until crisp. Remove bacon from skillet and drain all but 2 Tbsp. of bacon fat.
2. Cook chopped onion in bacon fat until tender. Add tomato soup, diced tomatoes, bacon and cooked spaghetti to skillet. Mix well to coat spaghetti in sauce and continue to cook until heated through. Sprinkle cheese over top of all.
3. Bake about 10 minutes or until cheese is melted and lightly golden brown.