Place the tortellini in plenty of boiling salted water and cook for about 15 minutes. Peel onion and garlic. Chop the onion finely, press the garlic through a garlic press. Heat olive oil in a high pan, fry onion and garlic in it.
Pour on broth and cream and add deep-frozen peas. Bring to the boil and simmer covered for about 5 minutes. Stir in sauce thickener, bring to the boil again briefly and season with salt and pepper. Pour tortellini onto a sieve, drain well and fold into the sauce.
Drain the tuna, also add and fold in. Arrange on deep plates and garnish with parsley.