Light a grill or preheat a grill pan. Lightly brush the grate or grill pan with oil. Grill the onion over the cooler part of the fire until lightly charred, about 5 minutes per side. Transfer to a plate. Grill the ribs over a hot fire until browned, about 45 seconds per side. Transfer to a work surface and cut into thick slices; discard the bones. Cover with foil. Toast the rolls on the grill or in the grill pan.