Cut steak into 4 serving-size pieces. Using a meat mallet, pound each piece to 1/8-inch thickness.
Spread one side of each beef piece with one-fourth of the mustard. Place 2 green pepper strips or 2 green onions at a short end of each beef piece and roll up. Secure with wooden toothpicks.
Lightly coat a cold large skillet with cooking spray. Heat skillet over medium heat. Add beef rolls; brown on all sides. Remove rolls from skillet. Add chopped onion to skillet and cook for 3 minutes or until onion is nearly tender.
In a small mixing bowl stir together beef broth and flour. Pour mixture into the skillet. Cook and stir until thickened and bubbly. Add beef rolls; cover and simmer 40 minutes. Add carrots; cover and simmer about 20 minutes more until beef and carrots are tender. Makes 4 servings.