Toss the pork with the sliced yellow onions, salt, pepper, and oil. Cover and refrigerate for at least 4 hours or overnight.
Light a charcoal grill and let the coals burn until evenly covered in pale ash.
Grill:
Discard the marinade onions. Thread the pork loosely onto flat skewers, leaving small gaps. Arrange the tomatoes, peppers, and eggplant near the cooler grill edges.
Grill the pork for 12 to 16 minutes, turning every few minutes, until browned and at least 145 F (63 C) inside. Grill the vegetables until softened and blistered.
Serve:
Rest the pork for 5 minutes. Peel the vegetables only where the skins are heavily charred.
Arrange pork and vegetables on a platter with red onion, herbs, lavash, and lemon wedges.
Notes
Pork shoulder stays juicier than lean loin over charcoal.
Keep a cool area on the grill so vegetables and meat can be moved away from intense heat.