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.
Pour off any excess oil and add the remaining sherry. Simmer until the alcohol has evaporated and you have a rich nutty 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 the remaining marinade from the tuna and the onion sauce to warm through.
Serve immediately with the parsley sprinkled on top and the lemon.