Place the tuna steaks on a board. Rub half the garlic into the fish, along with half the chopped fennel, fennel seeds, chilli and salt and pepper. Turn over and repeat on the other side.
Place the steaks side by side in a large flat container. Pour in the white wine, olive oil and the juice of 1 lemon. Cover and leave to marinate in a cool place for at least one hour.
Preheat a griddle or barbecue to very hot and sear the tuna for only a minute on each side.