In 12-inch cast-iron skillet, heat 2 tablespoons of the butter over medium-high heat. Add onions; cook 5 to 7 minutes, stirring occasionally, until onions begin to brown. Reduce heat to medium; cook 10 to 12 minutes, stirring frequently, until onions are browned. Increase heat to high. Add broth; cook and stir 5 to 7 minutes or until onions are dark brown and liquid has evaporated. Transfer onions to bowl.