Preheat the grill to medium-high heat
Peel the onions and trim the ends
Slice into thick rounds or cut into wedges
Keep the root end intact if cutting into wedges
Brush the onion pieces with oil
Season with salt and pepper
Place onions directly on the grill grates or use a grill basket
Grill for 4 to 6 minutes per side
Turn once or twice until tender and charred
Remove from the grill when softened and lightly browned
Serve immediately