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Slice the onions. Spread them apart with your fingers and place them on a rimmed sheet pan.
Drizzle the onions with the oil, vinegar, salt and sugar. Mix thoroughly.
Bake in a 400-degree oven for 40 minutes, stirring every 10 minutes. Remove from the pan and serve. These are great on salad, burgers, sandwiches or on top of steaks.