Preheat oven to 400 degrees. Place tomatoes, onion, garlic, salt, pepper and olive oil in 9x13 baking dish. Mix ingredients so vegetables are evenly coated in oil, then spread into a single layer in dish. Bake for 45-60 minutes, until juice is released from tomatoes and the garlic is soft (if tomatoes dry out during cooking, add 1/4 cup of water to dish as needed). Allow ingredients to cool, then puree in a food processor.
Chop or chiffonade basil, then stir into the sauce.