Select very large onions, 4 inches or so in diamater. Prepare onions by peeling off the papery skin, then slicing off about 1/2 inch from the top of the onions. Trim the root end, taking off the very least amount of flesh possible. Take a very thin slice from the root, just barely enough to help the onion remain standing on that end without toppling over.
Using a small, sharp knife and a spoon, hollow out the onions. Leave a wall of onion that is two layers thick at the top. Use the knife to cut into the top of the onion down into the center, leaving the outer 2 layers intact. Use the spoon to scoop the interior flesh out. Use care when you approach the bottom of the onion, the layers are thiner there and you may need to leave a bit more flesh.
Finely chop the onion trimmings from one onion. Discard the remaining onion trimmings, or reserve for another use. Set asidethe onion shells.
Heat oven to 350 degrees. Prepare a 9x9 baking dish by spraying with non-stick cooking spray.
Fry the sausage, crumbling into small bits as it cooks. When the meat is about halfway done, add the chopped onion and continue cooking until the meat is done and the onion tender.
Add the can of tomatoes, with their juice. Simmer a few minutes, until the tomatoes are heated through and the juice has blended into the meat.
Remove from heat and add the cheese, stir through.
Spoon the sausage mixture into the onion shells, packing down tightly. Place the filled onions in the prepared baking dish and place in oven. Bake for 60 minutes, or until the onions are tender and beginning to brown.
Sometimes I fill the onions, then refrigerate for another day. The cooking time is about the same. I have also used the same stuffing to fill bell peppers. The peppers bake for a slightly shorter time, about 45 minutes is usually long enough for them to be tender.