Add the onion, canned tomatoes (with juice), garlic, and enough water or broth to half-way submerge the vegetables. Cover and let gently simmer for 30 minutes.
Meanwhile, preheat oven to 400 F.
Using a slotted spoon, transfer the vegetables to a large baking dish. Pour liquid into dish, then top with sweet potato slices and rosemary sprigs.
Cover, leaving a slight tent for some moisture to escape. Bake for 30 minutes, until vegetables are tender and some of the liquid has cooked off.
Sprinkle with cheese and bake for 5 more minutes, to let melt.
Serve with your favorite pasta, or on a bed of fresh spinach. Top with chives and fresh parsley, if desired.