In a small bowl, mix together the mirin, Ponzu sauce, oyster sauce, and Worcestershire sauce until thoroughly combined.
Add 2 tablespoons of oil to the wok, along with the shiitake mushrooms. Stir-fry for 2 minutes and then add the onions, carrots, cabbage, and bell pepper.
Stir-fry for another 2 minutes, and then add the noodles. Pour the sauce mixture over the noodles. The liquid will help break them up.
Continue to stir-fry the mixture for another 2-3 minutes until the noodles are heated through. Add the scallions and stir-fry for 1 more minute. Serve, garnished with black sesame seeds if desired.