Skin the tomatoes. Dice the onion and roast in olive oil. Add salt and sweetener. Cut the skinned tomatoes into pieces above the pot with the roasted onions, skip the stalk and throw it away (above the pot so that the juice is caught and not lost), let it boil down. After about 10 minutes, whirl once with the hand blender so that everything is pureed very finely. Finally, season to taste as you like.
Can be used as a low-fat tomato sauce for pasta or served hot or cold as a soup. You can also add cream to the soup.