Thinly slice the beef and mix with black pepper and soy sauce. Let sit for at least 5 minutes.
Whisk together black pepper, potato starch, chicken stock, and soy sauce in a bowl.
Heat a wok with oil and stir-fry the marinated beef until browned. Remove and set aside.
Stir-fry bok choy until slightly softened. Meanwhile, cook udon noodles as per package instructions, then drain and add to the pan.
Return beef to the pan, pour in the sauce, and toss everything together until well coated.
Serve hot with extra black pepper or chopped green onions if desired.
