Heat vegetable oil in a heavy skillet over medium heat.
Stir sugar into vegetable oil with a wooden spoon, stirring constantly until mixture is a light caramel-color, about 10 minutes. Mixture will be very hot.
Stir onion, salt, and black pepper into sugar mixture. Onions will immediately start to caramelize. Stir frequently over medium heat until sugar has dissolved and onions are browned, about 15 minutes.
Pour red wine vinegar into onion mixture and simmer until mixture has a jam consistency, about 30 minutes. Let cool to serve.
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