Slice bacon into small 1/4 inch pieces. Slice onion thin. Cook noodles according to package. Fry bacon in pan and add onion about half way thru browning bacon. Do not drain! Pour all onto drained cooked spaghetti, bacon grease and all. Pour sauce and diced tomato into mix. I usually do this in the big pot I cooked the noodles in and keep it over a warm turned off burner. Salt and pepper to taste. Stir in the cheese until melted.
I usually serve with a green salad and some crunchy warm bread.
This has been a family recipe for 3 generations. My husband will take this any day over regular Italian spaghetti!