Heat a ridged grill pan or barbecue to medium, and have ready 10–12 bamboo skewers soaked in water (get these in a Chinese supermarket).
Slice the onions horizontally 1 cm thick; you should have 8–12 slices. Carefully insert the skewers through the side of each onion slice. This will hold all the rings together on the grill as they cook. Lightly brush with oil and season. Grill on one s