Combine the wine, lemon juice, garlic, rosemary, salt and pepper to make a marinade.
Place the tuna in the marinade for at least three hours - preferably longer.
Remove the fish from the marinade, drain off the excess liquid and fry quickly on both sides over a high heat - do not cook through, there is no need for it - all you need to do is to brown both sides quickly.
Mash the sardines into a paste, spread over the tuna and serve with plenty of lemon and good, crusty bread.