Heat an additional tablespoon
sesame oil in the wok over high heat. Add "stir fry" vegetables and saute for 1 minute. Add reserved sauce and bring to a simmer. Season with 1/2 teaspoon salt and 1/4 teaspoon pepper. Cook until sauce has thickened, about 1-2 minutes. Add beef back to skillet with vegetables and sauce and continue to cook until meat is cooked through and vegetables are crisp-tender, about 1 minute. Taste and add additional Gochujang/salt/pepper if desired.