Place a cast-iron skillet or other heavy ovenproof pan in oven; preheat oven to 450 degrees. Season chicken inside and out with salt and pepper.
Carefully remove skillet from oven; place chicken in skillet. Roast until juices run clear when chicken is pierced between breast and leg or an instant-read thermometer inserted into thickest part of a thigh (avoiding bone) registers 165 degrees, about 35 minutes.
Cover chicken loosely with foil and let rest 15 minutes. Shred meat into bite-size pieces. Heat each tortilla over a gas flame, or stack them, wrap in foil, and warm in turned-off oven 10 minutes. Fill tortillas with chicken, onion, cilantro, and avocado. Serve tacos with lime wedges.