Shred the cabbage. Cut the carrot and bell pepper into thin strips. Thinly slice the onion and shiitake mushroom.

Place the pork in a plastic bag, add the soy sauce and sake for marinade, and rub to coat. Sprinkle with cornstarch and toss to coat evenly.

Heat sesame oil in a skillet. Add the pork and cook until about halfway done.

Add the onion, then carrot, shiitake mushroom, cabbage, and bell pepper to the skillet in that order, stirring and adding a pinch of salt as you go.

Place the udon noodles in the center of the skillet, add the water, and gently loosen the noodles while mixing with the vegetables.

Add the soy sauce, mentsuyu, and oyster sauce. Stir to combine and finish.