Sanitize all work areas, cutting boards, equipment and sinks used in the preparation of this recipe.
Combine onions, oil, 1st portion of salt and black pepper into a stainless-steel bowl.
Gently toss together to combine and thoroughly coat the onions evenly with the marinade.
Place onions onto a preheated grill.
Sear evenly on both sides until charred, about 5 minutes per side.
Transfer onions, as they're charred, to a hotel pan.
Cover the pan with foil to seal tightly.
Cook in a preheated 325°F oven until tender, about 8-10 minutes.
Remove from the oven. Carefully remove the foil from the pan.
Carefully add vinegar and 2nd portion of salt.
Cover with plastic wrap. Let sit, allowing the onions to steam.
Remove the plastic wrap. Cool completely.
Transfer to clean food storage containers. Cover, label and date and store refrigerated.
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