Grind onion and tomatoes into fine paste separately, then fry them. Or first fry the onion and then grind them to make this process easier.
Leave it to simmer on the stove keep stirring in between when starts leaving the edge of the pan add pureed tomatoes to it and let the paste cook for 10-15 minutes more on the lowest heat setting.
Take it out of pan let it cool, portion it and store in the freezer and use it later.