Heat the oil in a 12-inch saute pan, add the onions and sugar, salt and pepper and red pepper, and toss to coat. Cook over medium heat until bottoms of onions are well browned, about 5 to 7 minutes. Add the vinegar, toss well, and cook on stove an additional 5 minutes, until vinegar begins to reduce.
If your saute pan is ovenproof, place it in the oven to roast. If not, transfer to a baking dish and roast in the oven. Cook until onions are fork tender, about 15 to 30 minutes, depending on the size of the onions.
Recipe by Allons-Eat! at https://www.allonseat.com/balsamic-roasted-cipollini-onions/