Pour boiling water over the tomatoes, peel the skin, cut them into a 1-2 cm / ¾-inch cubes.
Cut chili pepper into slices, finely chop the garlic. If you want the sauce to be less spicy, remove the seeds from the chili pepper. You can also use dried chili flakes, if you don‘t have fresh chilies.
Heat the olive oil in a large pot, add the garlic and chili, cook, stirring, for a minute.
Add the tomatoes, rice vinegar, Worcestershire sauce, and brown sugar. Cook for about 30 minutes over low heat. The sauce should be thick, stir it from time to time, keep an eye on it especially at the end of cooking time so that it doesn‘t burn.
Season with salt and pepper to taste, add potato flour, mix and bring to a boil, then take off the heat. Taste and add more salt, pepper, vinegar or sugar, if necessary.