You can always put them in a flowerbed. The flower heads are really pretty (I grew them for years and never had the heart to dig them up
.) That way, they won't contaminate his tomatoes
The tops are good for greens. For that matter, when you cook with onions, if you leave even a bit behind when you cut off the root end--the books say half an inch, but it doesn't have to be that much--, you can sock it in potting soil and you get another onion. As long as you keep giving it dirt to make more onion out of, you can keep using the plant.