Heat oven to 350°F. In 10-inch skillet, melt butter over medium-high heat. Add steak strips; cook 8 to 10 minutes, stirring frequently, until no longer pink. Remove steak from skillet; place on plate. Add bell pepper and onion to skillet; cook about 5 minutes, stirring occasionally, until crisp-tender. Return cooked steak to skillet; mix well. If desired, add salt and pepper to taste.