In a small bowl whisk together peanut butter, lime juice, soy sauce, and cider vinegar.
In a large bowl toss together cooked lo mein noodles, leftover stir fry vegetables, scallions and cilantro. Toss with dressing until all the noodles are well coated.
Stir-Fry: In a wok or large skillet over high heat add the canola oil. Add the beef and cook until browned, about 3 minutes. Set aside on a plate.
To the same pan the beef was cooked in, add the carrots, celery, onions, mushrooms, ginger, garlic, and broccoli and stir-fry until slightly tender but not soft, about 3 minutes. Remove half the vegetables and reserve for the online round 2 recipe: Cold Noodle Salad. Add the beef back to the skillet along with half of the hoisin sauce, and toss to coat the beef. Reserve half of the lo mein noodles for the round 2 recipe: Cold Noodle Salad. Arrange the remaining half of the lo mein noodles on a serving platter and top with the beef stir-fry.
Hoisin Sauce: In a medium bowl, whisk all the sauce ingredients until well blended. Add half to the stir-fry and save the remaining half for the next time you make a stir-fry. The extra hoisin sauce will last up to 2 weeks in a sealed container stored in the refrigerator.