In 10-inch skillet, heat oil and 1 tablespoon butter over medium-high heat. Add onions; cook about 5 minutes, stirring frequently, until onions begin to brown. Reduce heat to medium-low; cook 15 to 20 minutes, stirring frequently, until onions are golden brown. Add balsamic vinegar, sugar and salt; cook and stir 1 to 2 minutes or until onions are dark brown. Transfer onions to bowl; cool 10 minutes.