Make the marinade: Heat a medium skillet over medium. Add the garlic cloves and onion, and cook on one side until charred in spots, about 5 minutes. Flip and cook until charred on the opposite side, about 5 minutes more. Transfer to a blender.

Place the chiles in the same skillet and press down firmly until blistered and fragrant, about 1 minute. Flip and repeat on the other side. Add to the blender.

Turn off the heat under the skillet and immediately add the coriander and cumin. Swirl the pan until the spices are fragrant and the cumin sounds like it's popping, 15 to 30 seconds. Transfer the spices to the blender, then add the grapefruit and lime juices, 1 tablespoon salt, annatto, oregano and paprika. Blend until smooth.

Transfer the marinade to a bowl, or resealable freezer bag, and add the chicken. Toss to coat and transfer to the fridge for at least 3 hours and up to 12 hours.

Using your hands, scrape off excess marinade. With a brush, coat the chicken with some neutral oil and season on all sides with salt. Place on a baking sheet or tray for carrying to the grill.

Prepare a grill for indirect heat: Heat the coals on one half of a charcoal grill, or turn half the burners to medium-high on a gas grill. Lightly grease the grates of the grill with a paper towel dipped in neutral oil and place the chicken skin-side up on the side of the grill without coals (or turned on burners). Cover the grill and cook until chicken registers 165 degrees on an instant-read thermometer, 25 to 30 minutes. If you’d like more color on the skin, flip the chicken and place skin-side down on the coal side with the cover open, until it’s nicely charred. Be vigilant, as the chicken can quickly burn; it will only need 1 or 2 minutes. (Alternatively, cook the chicken on a grill pan over medium heat, flipping the pieces every 5 minutes, until the chicken registers 165 degrees.)

Transfer the chicken to a platter, season with salt, tent with foil and let rest for at least 5 minutes before serving with cilantro and lime wedges.