Melt butter in a large skillet over medium-high heat. Add onion; cook 5 minutes, stirring frequently. Continue cooking 15 minutes or until deep golden brown, stirring constantly. Add wine; cook 5 minutes or until liquid almost evaporates. Remove onion from pan; finely chop onion. Return onion to pan; stir in broth, salt, and pepper. Bring to a boil; cook until reduced to 2 cups (about 10 minutes).