Slice the onion, carrot, cabbage, and green onion into thin strips.
Cut the beef (or pork) into bite-sized pieces.
Heat oil in a pan and stir-fry the meat first.
When the meat is about halfway cooked, add the prepared vegetables and stir-fry together.
Blanch the udon noodles in boiling water, then add to the pan.
Season with oyster sauce, soy sauce, and pepper.
Stir everything well until combined and cooked through.