In a large saucepan, heat the oil over medium heat. Add the onion and garlic and cook until translucent. Add the diced fresh tomatoes, broken plum tomatoes (along with juices) and bring to a simmer. Cook until tomatoes release their juices and begin to break down, about 20 minutes.

Add the basil, sugar, and salt and pepper to taste and continue to cook until the sauce reaches the desired consistency and any raw flavor is gone. If sauce seems too thick (depending on the tomatoes), you can add a bit of water if necessary.