Heat a large nonstick skillet over medium-high heat until hot; add frozen pepper stir-fry, and cook 2 minutes. Add beef strips, and cook 3 to 5 minutes or until thoroughly heated; drain.
Spread 1 teaspoon mustard blend on bottom half of each roll. Spoon 1/2 cup beef mixture onto each roll; top with cheese and top half of roll. Wrap each sandwich in aluminum foil. Bake at 350° for 6 to 7 minutes or until cheese melts. Remove sandwiches from foil; serve immediately.