Heat the oil in a saucepan over medium heat. Add the tomatoes, onions, and pepper. As the tomatoes give up some of their liquid and it starts to boil, lower the heat and cook the vegetables until they are quite soft, about 45 minutes.
Transfer the vegetables to a vegetable mill and purée them. Return the purée to a clean saucepan and add the remainder of the ingredients. Bring to a simmer and cook for an additional 45 minutes or so to thicken the mixture.