Cut tomatoes into small pieces. Cook tomatoes, using only ¼cup water at the beginning and only adding more if necessary. Cook for about 1 hour over very, very low heat. If tomatoes are completely puréed, sometimes you may have to cook them for a longer time.
Force tomatoes through a sieve (do not use the blender for this!) to remove skin and seeds.
Add black pepper and ground cherry pepper. Drop by drop whip in the oil till completely absorbed.
Finally, mix in the minced (almost puréed) onion, and let the sauce cool.