First, make the sauce. Add ½ tbsp oyster sauce, ½ tbsp Japanese soy sauce (koikuchi shoyu), 1 tsp Worcestershire sauce, ½ tsp tomato ketchup, 1 tsp sake, ¼ tsp sugar, 1 pinch ground black pepper and ½ tsp toasted sesame oil to a small bowl. Mix well and set aside for later.
Heat a wok or frying pan on medium high and add ½ tbsp cooking oil. Add 100 g yakisoba noodles (pre-steamed) and fry until slightly crispy on both sides.
Transfer the noodles to a plate and add 60 g thinly sliced pork belly to the same wok with 1 pinch salt and pepper. Fry until crispy.
Add 25 g onion to the wok and stir fry until it reaches your preferred softness.
Push the meat and onion to one side and add the noodles back to the pan. Pour your yakisoba sauce over the noodles only (not the pork and onions) and mix until they're coated.
Mix everything together and stir fry for another minute or so before removing the wok from the heat.
Cut 3 hot dog buns through the middle and spread each side with butter. Place an equal amount of yakisoba in each bun and top with aonori (dried green seaweed powder) and red pickled ginger (benishoga).
Enjoy!
Notes
If you can't find thinly sliced pork belly, you can use unsmoked bacon instead. I recommend 1 slice per portion.
You can add more vegetables if you plan to eat it straight away. Cook all vegetables after the pork, with the firmest first (e.g. carrots) and the softest last (e.g. beansprouts). Don't add the noodles back to the pan until the vegetables are cooked through.
Be aware that adding extra ingredients will make the yakisoba more bulky and you might need extra hot dog buns.
If you're serving yakisoba pan in a lunchbox or saving it for later, I don't recommend adding extra vegetables since they can make the bread soggy. Wrap yakisoba pan in clingfilm for later and eat within a few hours.