Preheat the oven to 400°F. Place the tomatoes, cut side up on a large baking tray and add 1 clove garlic and the onion. Arrange the tomatoes, garlic and onion so they do not touch each other. Drizzle 1 tablespoon oil over all and roast for 30-35 minutes, or until very tender. Remove and cool slightly.
Place the cooled vegetables, remaining garlic and oil in the blender and add the salt and pepper to taste, along with the basil and oregano. Select Auto 5 and blend until just chunky-smooth.
Heat the sauce in a 3 quart saucepan over medium heat, stirring occasionally, until warmed throughout. Serve over hot pasta, grilled chicken or fish.