Mince and mash the garlic.

In a shallow dish, whisk together garlic, 2 tablespoons lime juice, 3 tablespoons olive oil, red pepper flakes, and salt to taste.

Add the chicken to the marinade, turning to coat.

Let marinate, turning once, for 20 minutes.

Dice the avocado into 1/4-inch pieces.

In a small bowl, combine avocado, remaining 1 tablespoon lime juice, remaining 2 tablespoons oil, red onion, coriander, and salt to taste.

Toss the salsa until combined.

Heat a ridged grill pan or cast-iron skillet over medium heat.

Cook the chicken skin side down, covered, for 4 minutes.

Turn the chicken skin side up and cook, covered, for 2-3 minutes more, or until firm but springy to the touch.

Transfer the chicken to a cutting board.

Holding the knife at a 45-degree angle, cut each half crosswise into 1-inch slices.

Transfer the chicken to heated plates.

Garnish each plate with red onion slices, lime wedges, and chili pepper (if desired).

Top the chicken with the salsa.

Serve immediately.