PREPARATION: Prepare a charcoal or gas grill for direct grilling over medium-high heat. Oil the grill racks with paper towels saturated with a neutral oil such as canola.
MARINATE THE TUNA: Whisk together the marinade ingredients in a measuring cup. Place tuna steaks in a non-reactive container (glass, stainless steel, or plastic) and pour half the marinade over the fish. Flip the fish and pour the other half of the marinade on top. Let fish rest in the marinade while grill heats.
GRILL THE TUNA: Grill the tuna directly over the heat elements, turning once, for 3-5 minutes on each side until it is seared on the outside and either rare or pink on the inside. (Cook a little longer if you like your tuna done.)
PLATE THE TUNA: Add each fillet to a serving plate and top with 3-4 tablespoons tapenade. Serve hot.
Cover tightly any leftovers, refrigerate quickly, and eat within a day or two