Cook the pasta in sparkling salted water as dente.
Meanwhile, dice the onion, chop the garlic very finely or press it through the press. Sauté both together in the butter over medium heat until translucent. Add the drained tuna and fry briefly. Add the juice of half a lemon, mix everything together and simmer briefly. Finally stir in the cream and capers. Do not boil any more, just let it warm up briefly. Season to taste with pepper, parsley and, depending on your taste, with the remaining lemon juice. Mix with the pasta in the pan and arrange on plates. Garnish with a slice of lemon and a sprig of parsley.
A tomato salad with balsamic dressing goes perfectly with it.