Cut the tuna into 2cm steaks and marinate them in the vinegar, garlic and bay. leave for 1 hour.
Place a pot on the heat with the olive oil, lightly sear the steaks on both sides and remove from the pan. add the onion to the pan until it turns translucid. add the steaks again and the marinade. cover and cook for 3 minutes.
Serve with chopped parsley, esparregado and chips.