2 tablespoons olive oil
2 large onions, cut into 1/8-inch-thick slices
1/4 teaspoon salt
1/8 teaspoon ground black pepper
1/3 cup canned beef broth
1/2 cup purchased barbecue sauce
1 1/2 tablespoons apple cider vinegar or white vinegar
Heat oil in a large skillet over medium-high heat.
Add onions, salt, and pepper; cook until onions begin to brown, stirring frequently, about 8 minutes.
Add broth, barbecue sauce, and vinegar.
Reduce heat to medium; simmer until sauce thickens, stirring occasionally, about 15 minutes.
Note: Can be made 1 day ahead. Cover; chill. Rewarm before serving.