Marinate tuna steak or fish fillets with olive oil, roasted garlic, ginger, thyme, cilantro, salt, and black pepper. Refrigerate 1/2 hour. Combine Mango-Serrano Pico ingredients, mix well and refrigerate until ready to serve. Prepare and heat grill. Lightly brush steaks with oil before placing on hot grill. (To prevent steaks from sticking, lightly oil the bars of the grill with vegetable oil by dipping a tightly folded paper towel in oil and, holding it with long-handled tongs, rub it over the grill - be very careful not to burn your hands!) Grill fish on medium to hot grill, for about 3-5 minutes per side, rotating each steak a quarter turn after 1 1/2 minutes on each side to create attractive grill marks. Serve grilled tuna topped with Mango-Serrano Pico.