Cook the pasta in boiling salted water for about 10 minutes until al dente
For the marinade, peel and finely dice the onion. Bring to the boil with 4-5 tablespoons of water, vinegar and stock. Season with salt and pepper
Dice mozzarella. Drain the pasta. Mix both with the marinade. Let stand for at least 30 minutes.
Wash the tomatoes and cut them into pieces. Wash lettuce leaves, shake dry and pluck smaller. Mix both into the noodles, season again. Sprinkle with cress