In a medium, heavy-bottom saucepan, stir together the oil, onion and garlic. Cook over low heat, stirring often, until the onion is very soft and aromatic, about 6 to 8 minutes. In a food processor or blender, purée the tomatoes. Add the tomatoes and tomato paste to the onions and bring to a simmer over medium-high heat. Reduce the heat to very low and let the sauce simmer slowly for 30 minutes, stirring the bottom often to prevent burning. If you are adding meatballs, do so at this time, and simmer them in the sauce for 20 minutes, stirring often. If you are not adding meatballs, simmer the sauce for another 20 minutes (50 minutes total). Season to taste with salt and pepper. Remove bay leaves before serving.