For the ketchup, peel and chop the tomatoes. Cut the onion into small pieces. Heat a little oil in a saucepan, sauté the onion, add the tomatoes and sauté for about 5 minutes.
Add salt, vinegar and sugar. Simmer gently for about 5 minutes, pass the mixture through a sieve and put the ketchup in a sterile container.
Preparation Tip:
The ketchup keeps refrigerated for about 1 week.