Very finely chop the tomatoes, being careful to reserve the tomato juice. In a large, deep, heavy skillet, heat the sunflower oil over medium-low heat. Add the garlic and onion (if using), and cook until very soft, stirring all the while, about 15 minutes. Add a pinch of salt. Pour the tomatoes into the pot, turn the heat up to high, and bring to a rolling boil. Reduce the heat to low and simmer. Sprinkle in a pinch of salt, and stir. Cook, stirring once in a while, for at least 90 minutes. During the cooking time, the tomatoes will gradually become thicker, first turning into a thick sauce, and then it will get sticky. As it gets thicker, stir more frequently. When sticky, stir even more, scraping the bottom of the pot. At a certain point, the oil will look like it's separating from the tomatoes - keep stirring, and suddenly the tomatoes will stick together like a dough. You've made tomato paste! Transfer to a bowl and allow to cool. When cool, place in small jars or other containers, press a layer of plastic wrap onto the surface. The tomato concentrate will keep in the refrigerator for weeks. For longer storage, keep in the freezer.