First, peel the onion and cut into thick rings or strips.
Prepare a marinade from balsamic vinegar, olive oil, salt and pepper.
Put the onion rings in a container, pour the marinade over them, cover with foil and let the whole thing infuse for about 30 minutes.
Lift the onions out of the marinade, place them on a grill tray and grill on both sides for about 5 minutes.
Drizzle the grilled onions with a little marinade before serving.