Drain the tuna. Peel and finely chop the onion and garlic. Fry in hot oil. Dust with flour and sweat lightly. Stir in cream, 150 ml water, stock and frozen peas. Bring to the boil and simmer for about 5 minutes.
Season to taste with salt and pepper.
Cook the tortelloni in plenty of boiling salted water according to the package instructions. Then drain and fold into the sauce with the tuna. Serve.