Use this to make our Stuffed Beef Burgers.
Melt butter with oil in a medium skillet over medium-high heat. Add onions, salt, and pepper. Cook, stirring often, until edges of onions turn golden, about 5 minutes. Reduce heat to medium-low. Cook until onion is softened and just golden brown, about 5 minutes.
Transfer onion mixture to a medium bowl. Let cool completely. Stir in bacon and cheese.
Filling can be refrigerated in an airtight container, up to 1 day.