Clean and wash the leek and cut it lengthwise into fine strips. Boil up plenty of salted water. Cook the noodles for about 12 minutes. Add the leek to the noodles shortly before the end of the cooking time.
Peel and finely chop the onion. Heat the fat in a pot and fry the onion. Add cream and stock, bring to the boil. Drain capers and add to the sauce. Stir in the sauce thickener and bring to the boil.
Drain the tuna, pluck it into pieces and heat it in the sauce. Season the sauce with salt, pepper and lemon juice. Drain the pasta and let it drain. Arrange pasta and leek with the sauce on plates.
Garnish with lemon wedges and caper apples.