Serrano chiles are hotter than jalapeños, and this salsa is very hot, so adjust the amount to your preference. Serve this dish with Nopales-Arugula-Roasted Pepper Salad. Prepare the salsa up to 2 hours before serving; if you do so more than an hour before, refrigerate and bring to room temperature before serving.
Place the avocado, tomatillos, cilantro, roasted garlic, vinegar, water and chiles in a blender; purée just until smooth. Transfer to a bowl and fold in onion and salt. Set aside.
Heat oven to 350˚F.
Sprinkle the chicken with salt and pepper.
Heat the oil in a large ovenproof sauté pan or skillet over medium-high heat. Add the chicken legs with skin side down and sear for 1–2 minutes until golden-brown. Flip the chicken, transfer skillet to the oven and bake uncovered for 15-20 minutes until just done. (Juices will run clear.)
Place one chicken leg on each plate and spoon one-quarter of the salsa around each plate and on the chicken. Garnish each with a sprig of cilantro.
*To roast garlic, toss the cloves in just enough olive oil to coat and place in a square of aluminum foil with about 1/8 cup of water. Fold and crimp foil pouch to close and place in a preheated 350°F oven for about 40 minutes.
**Wear rubber gloves when handling chile peppers and be careful not to rub your eyes.