Peel the onions and cut into centimetre slices. Keep the slices intact when you put them on the grill on a medium-hot griddle (or on a Teflon pan at home).
Mix the marinade ingredients in a bowl.
Grill the onions for a few minutes and flip, the onions should be slightly charred.
Stir the onions into the marinade straight from the grill. Let cool and stir a couple of times while cooling.