Heat 2 tablespoons of the oil in a frying pan. Add the onions and cook over a low heat for 15–20 minutes until soft and starting to turn golden. Add the garlic and cook for 1 minute. Pour over the sherry and simmer for a few minutes until you have a rich sauce. Remove from the pan and set aside. Wipe the pan clean with kitchen paper.
Heat the remaining oil in the pan and season the tuna steaks well. Add the tuna steaks to the pan and cook for 1–2 minutes on each side until lightly browned. Pour over the onion sauce, then scatter with the parsley and serve immediately.