Preheat the oven to 375°F. Line a baking sheet with parchment paper (this helps to easily transfer the tomatoes to the blender).
Place the tomatoes, onion, and garlic on the baking sheet. Season with salt and pepper. Drizzle with the oil or water. Roast in the oven 1 hour.
Remove from the oven and let cool 5 minutes. Then transfer the tomatoes to a blender. Add the basil and oregano. Blend until smooth. Taste and season with additional salt and pepper, if desired.
Notes
Yield: 5 cups (about 2 store-bought jars worth)Nutrition: nutrition facts are calculated with the oil included.If desired, you can thicken the sauce: transfer the blended sauce to a large skillet or pot with a lid. Bring to a simmer then turn the heat to low, cover partially, and cook for 30-60 minutes.If you plan to cook meatballs in the sauce, do not follow the thickening instructions above and instead add the blended sauce to a large skillet, along with the meatballs, and cook according to the recipe instructions for the meatballs.