Clean, wash and cut the leek into pieces of about 12 cm. Cook in boiling salted water for about 8 minutes, then drain. In the meantime peel and chop the onion.
Heat the oil in a frying pan. Fry the onion and minced meat in it while turning. Season with salt, pepper and stock. Cut the tomatoes roughly while still in the tin. Deglaze the mince with it. Fill the tin with 3/8 litres of water and also pour it into the mince.
Let it boil down for 5 minutes at high heat. Meanwhile boil up 3/8 litres of water and 1/2 teaspoon of salt. Remove from the heat. Add milk and fat. Stir in the puree flakes. Leave to swell for 1 minute and stir again.
Cut the mozzarella into slices. Fill leek pieces and minced sauce into portion moulds or a large mould as desired. Cover with mozzarella. Bake in a preheated oven (electric range: 225 °C/ convection oven: 200 °C/ gas: level 4) until golden brown.
Serve sprinkled with parsley.