Bring a large pot of water to a boil. Cut the stem end out of each tomato. Cut a small, shallow X into the blossom (curved) end of each tomato. Plunge the tomatoes, in batches, into boiling water for 1 to 2 minutes. Lift out of the water and set in ice-cold water. Pull the skins off the tomatoes (they should slip off easily). Cut each tomato in half, remove and discard the seeds, then coarsely chop and place in a bowl. Add any liquid from the cutting board to the bowl.
Heat the oil in another pot set over medium heat. Add the onion, bell pepper and garlic and cook until tender, about 5 minutes. Add the tomatoes, stock, tomato paste and sugar and bring to a gentle simmer. Gently simmer until the tomatoes are very tender and falling apart, about 20 minutes. Stir in the basil, salt and pepper.
Purée the mixture in the pot with an immersion blender, or in a blender or food processor. Use immediately or cool to room temperature before freezing in amounts to use as needed.
Options: Instead of on-the-vine tomatoes, make this sauce with an equal weight of plum (also known as roma) tomatoes. Instead of fresh basil, use 1/3 cup of fresh oregano leaves in the sauce, or use a mix of the two herbs. For a spicy taste, add crushed chili flakes to taste when sautéing the onions and peppers.