Start out sautéing the minced garlic in about 1 tbsp of olive oil in a small sauce pan on medium heat. Cook for about 2 minutes – enough to turn the garlic translucent but not start browning.
Add the chicken stock, wine, and lime zest. Bring to a boil, then reduce heat and simmer for 10 minutes.
Add in the lime juice and remaining olive oil and continue simmering for another 3~5 minutes. Things should be starting to thicken up at this point. Turn off the heat and add in the cilantro, mixing it in well. Let sit for a minute.
Place the fish in a flat shallow bowl (or a deep plate). Spoon some of the marinade mixture over the fish, then turn and cover the other side as well. Make sure to get plenty of garlic and cilantro on both sides of the fish.
Reserve the remaining marinade, returning the pan to the stove and turning it on very-very low to keep it warm but not quite at a simmer. We’re trying to promote further evaporation of the liquid without deteriorating the delicate flavor of the cilantro (which disappears quickly when cooked).
Let the fish sit in the marinade (about 15 minutes) while you preheat the grill (you can also bake this if you wish) to 400 degrees indirect heat.
Make sure to turn the fish once or twice while things are warming up, and get your side dishes started.
Cooking time will of course depend on the thickness of your fish and how “done” you like it. Tuna is best when cooked to medium-rare or medium, so I found 8 minutes per side (turning once) on the grill to be about right for these thick fillets (they are about 2-1/2″ to 3″ square in cross-section). For thinner cuts, adjust the time appropriately so it finishes flaky with a fork but still moist.
Serve with the reserved warm sauce drizzled over the top. Enjoy!