Preheat the oven to 400°. Remove skins from the onions. Lay them in oven-proof baking dish along with the tomatoes. Drizzle with olive oil. Sprinkle with thyme, rosemary, salt, and pepper.
Pour the balsamic vinegar over the onions and tomatoes. Roast them for approximately 15 minutes and then turn the temperature down to 350° and bake for another 15 minutes. You need to watch the onions/tomatoes so they do not brown too quickly. You may want to turn the temperature down to 350° sooner. If the onions don't look done and tender, continue to cook until they are soft to touch.