Fry the onions and garlic in a little oil until translucent. Add tomato paste and fry briefly.
Deglaze with the canned tomatoes and allow to boil down. Add the lentils, vegetable stock and the water and simmer for about 15 minutes. Drain the tuna, pluck it into small pieces and add it with the grated cheese 5 minutes before the end of the cooking time. Season, season to taste and a great pasta sauce is ready.