A quick, five-ingredient Red Onion Jam heightens the flavor of cheesy homemade burgers .
Heat oil in a heavy skillet over medium heat. Add onions, sugar, and salt; sauté 25 minutes or until onions are softened and lightly caramelized.
Stir in vinegar; reduce heat to low, cover, and simmer 8 minutes or until mixture thickens. Let cool before serving.