In a deep saucepan, bring 6 cups of water to a boil over high heat and add the tomatoes. Boil for 5 minutes, or until the skin begins to peel off.
In the pitcher of a Vitamix or high-speed blender, place the tomatoes, garlic, and onion. Blend on medium speed for 30 seconds. Then add water in 2-tablespoon increments and blend again to reach the desired consistency.
Pour the blended sauce into a saucepan over medium-low heat and add the olive oil. Heat until warm, about 5 minutes. Add the salt and pepper and adjust to taste.