Finely chop the onion. Drain the tuna and crumble into pieces. Wash the lemon with hot water, dry it, rub off the peel and then squeeze out. Rinse capers. Finely chop the parsley. Skin and dice tomatoes.
Cook the spaghetti in salted water according to the instructions.
Heat the butter and fry the onion with the lemon zest. Add the tuna with the capers and fry briefly. Pour lemon juice into it and add diced tomatoes, allow to evaporate a little.
Stir in the cream, salt, pepper and parsley and remove from the heat.
Pour off firm spaghetti, serve with the sauce and garnish according to your own preferences.