Preheat an outdoor grill for high heat and lightly oil grate.
Peel onions and trim root ends just slightly. Quarter the onions, keeping the layers intact with the remaining root ends. If necessary, use a toothpick to keep layers together while on the grill.
Grill onions until slightly charred, about 5 minutes.
Remove from the grill and place in a nonreactive bowl.
Meanwhile, heat vinegar, sugar, and salt in a small non-reactive saucepan over medium heat. Stir until the sugar is dissolved.
Pour hot liquid over the grilled onions and let stand for 10 to 15 minutes.
Remove onions from liquid and refrigerate until ready to use.
