1. Take a bowl and mix sherry, olive oil, lemon juice, soy sauce, gingerroot, garlic and pepper well to prepare the marinade.
2. Take a 12x7 1/2x2-inch baking dish and arrange tuna steaks as a single layer and pour the marinade.
3. Cover, keep in the refrigerator and allow the tuna to marinate for 1 to 2 hours.
4. Turn tuna steaks intermittently for even marinating.
5. Drain the tuna steaks and discard the leftover marinade.
6. On a cooking grid, arrange the marinated tuna steaks.
7. On medium coals, grill directly uncovered for 10 minutes or till the tuna can be flaked off using fork.