Peel the onions, chop them finely and fry them in hot olive oil until translucent. Drain the tuna and add to the onions. Chop up a little in the pot and fry. Season with pepper and salt. Add sour cream and stir everything until a creamy sauce is formed. Season to taste with vegetable stock and cayenne pepper. Finally add tomato paste to taste and stir.
Mix with the pasta prepared according to the instructions on the packet and tossed in a little butter and serve.