Heat a cast-iron skillet or griddle. Add the garlic and toast over moderate heat until the skin is blackened in spots, 10 minutes.
Meanwhile, toast the pasillas in the skillet: Press with a spatula and turn once, until pliable and fragrant, 1 minute. Transfer the chiles to a heatproof bowl and cover with hot water. Microwave at high power for 1 minute, until the chiles are completely rehydrated.
Transfer the chiles to a blender and add 1/3 cup of their soaking liquid. Peel the garlic and add to the blender with the honey, vinegar, oregano, cumin, 1 teaspoon of salt and 1/4 teaspoon of pepper. Puree, then strain.
Rub the lamb with 1/3 cup of the puree. Let stand for 2 hours or refrigerate overnight.
In a small saucepan, simmer the remaining puree with 1/4 cup of water; keep warm.
Light a grill and oil the grates. Grill the lamb over moderately high heat, turning and rotating the racks, until lightly charred outside and an instant-read thermometer inserted in the center of the meat registers 135° for medium-rare, 30 minutes; move the meat to moderate heat if it starts to burn. Transfer the lamb to a carving board; let rest for 5 minutes. Carve the lamb into chops and garnish with chopped red onion, parsley and queso añejo. Serve with the pasilla sauce.
Pasilla chiles are long, black dried chiles. Queso añejo is a dry grating cheese; cotija or ricotta salata are good substitutes.