In 12-inch skillet, heat butter over medium heat. Add yellow onion and serrano chile. Cook 4 to 6 minutes, stirring occasionally, until vegetables are softened. Add flour and taco seasoning mix; continue to cook 1 minute, stirring constantly. Beat in broth with whisk; heat to boiling. Reduce heat to medium-low; simmer 2 to 5 minutes or until slightly thickened.