Slice or dice onions to preferred size and cook in sauté pan over Medium-High heat for 6-8 minutes or until desired.
Cut onions into ¼ to ½ inch slices and brush with oil. Place on grill over Medium-High heat until char marks appear, about 2 minutes. Use cooking utensil to flip so that rings don’t fall apart. Cook longer for softer onions.
Wash and peel onions. Chop raw onion into ½ inch segments and place in freezer bag. Lay onions flat in bag and seal airtight before placing in freezer.