Step 1 Place the tomatoes, onion, garlic and 1 1/2 cups of water in a saucepan and cook over medium-low heat for 1 hour, stirring occasionally.
Step 2 Then puree the mixture in a blender or food processor or with an immersion blender. Pour the sauce through a fine-mesh sieve to remove tomato seeds and skin.
Step 3Taste and add up to 2 teaspoons sugar as needed (fully ripe tomatoes should not need it).
Step 4You should have 2 cups of tomato sauce. If you have more, return it to the saucepan and reduce to 2 cups, stirring occasionally.