In same skillet, melt 1 tablespoon butter over medium-high heat. Drain beef using a strainer, discarding marinade. Add sliced onion & mushrooms to pan; cook and stir until onion is softened. Add beef & cook until meat is no longer pink, 6-8 minutes. Add mustard sauce; reduce heat to low and simmer, uncovered, until slightly thickened. Keep warm until serving.