Place the tomatoes, onion, and garlic on a baking sheet, drizzle with the olive oil, and season with salt and pepper.
Roast for 20 minutes, or until the veggies are softened and lightly browned.
Squeeze the roasted garlic cloves out of the peel. Place the roasted garlic, tomatoes, onion, Italian seasoning, and basil in a blender or food processor. Blend until the sauce reaches your desired consistency (the longer you blend, the smoother the sauce).