Peel and finely dice the onions. Heat the fat in a saucepan. Fry the onions in it. Dust with flour. Add tomatoes, apple juice and stock. Bring everything to the boil and cook for 5 minutes. Clean and wash the leek and cut it into thin rings.
Heat 2 tablespoons of oil in a frying pan. Sauté the leek in it and take it out. Heat the rest of the oil and fry the meatballs in it until brown all around. Puree the tomato soup with the cutting stick of the hand mixer.
Add leek and meatballs and heat again. Season soup with salt and Tabasco.