1 tablespoon kosher salt (halve if using table salt)
1 teaspoon coarse ground black pepper
1/4 cup olive oil
Use a vegetable peeler to peel the zest off the lemon. Use a sharp knife to cut the zest into long thin strips.
Rub the tuna steaks with the garlic, and then sprinkle both sides with the salt and pepper. Put the steaks in a large freezer bag, and then scatter the lemon zest across them. Drizzle the olive oil all over the tuna, and seal the bag, pressing out as much air as you can. Put the tuna in the fridge overnight to marinate.
When you are ready to grill the steaks, take them out of the fridge while you heat up the grill (about 15 minutes). The grill should be very hot when you put them on. Grill one side until you see the red tuna turn beige about 1/3 of the way up the side. Flip, then grill the other side until you can only see a hint of pink from the side.