Carve tomatoes crosswise at the top, put them briefly in boiling water, rinse with cold water, peel skin. Cut tomatoes into cubes. Peel and chop onion. Heat fat in a pot.
Sauté diced onions. Add tomatoes and stock and let everything boil down at medium heat for 15 minutes. Season to taste with salt and pepper. Wash parsley, chop finely and pour over the tomato sauce.