Heat a pan, preferably heavy-bottomed or enameled cast iron skillet, over medium heat with the olive oil. Finely chop the onion, carrot and celery and add to the olive oil. Sautèe the soffritto, stirring occasionally for 15-20 minutes until the vegetables are translucent.
If you have decided to use garlic, add it to the last two minutes of cooking. If you are adding red chili flakes, add them to the last minute of cooking.
Add the canned tomatoes with 1 cup of water and bring to a simmer. Gently simmer over low heat, uncovered until thickened and dark red, stirring every so often. This can take anywhere from 1 hour but up to 3 hours. The longer you cook it the better (Italians cook their sauce all afternoon if they have time). Add extra water if you notice the sauce is sticking to the bottom or becoming too thick. The amount of water you add will depend on your heat source and for how long you are cooking the pomodoro sauce.
Add 10-15 fresh basil leaves in the last 15 minutes of cooking.
If you like a smooth pomodoro sauce, strain the pomodoro sauce with a food mill. If you prefer a more full-bodied pomodoro sauce, leave as is and either freeze, can or refrigerate when cooled to room temperature.