First marinate the tuna. Place the tuna in a dish and rub the garlic all over. Pour over a third of the sherry and leave to sit for an hour in the fridge before cooking.

Meanwhile, place a large frying pan over a medium heat and add the olive oil. When hot, add the onion and a pinch of salt. When the onions have wilted, turn down the heat and continue to cook for a good 20-30 minutes, stirring occasionally, until they are golden brown and sweet in smell and taste. Pour off any excess oil, add the remaining sherry, and simmer until the alcohol has evaporated and you have a nutty, rich sauce. Set aside.

When ready to cook the tuna, heat a clean frying pan until very hot. Add a drizzle of oil followed by the seasoned tuna, and fry briefly on both sides, depending on how rare you like it and the thickness of the cut. When almost cooked, add both the marinade and the onion sauce to the pan to warm through. You could use a griddle pan instead and warm the sauce separately. Serve immediately with the parsley sprinkled on top and the lemon.