Pour the soup into a medium saucepan and cook over medium heat for 10 minutes. Taste for seasoning. Halve the tomatoes. Put the tomatoes in a shallow baking dish in a single layer. Drizzle with olive oil.
Scatter onion slices over the top, then add garlic.
Bake for 45 minutes to an hour. Pass the baked vegetables through a food mill to remove seeds and skins. Season with salt, pepper and fresh or dried herbs of choice. Use as a sauce for pasta, vegetables, fish, baked chicken and so on. Makes about makes about 2 quarts.