Massage the soy sauce into the pork to pre-season it. Chop up the cabbage, and slice the carrot into short rectangular pieces. Slice the leek and bell pepper thinly.
Heat some sesame oil in a frying pan and stir fry the pork over medium heat. When the color of the meat changes, add the leek, bell pepper and carrot and stir once, then add the cabbage on top (don't mix it in). Put a lid on and steam cook over low heat.
After about 5 minutes the cabbage will have wilted and moisture will be coming out of it. Turn the heat up to medium and add the udon noodles. Mix well, add the ketchup and sauce, and it's done Serve with aonori and bonito flakes on top if you like.