The idea of a noodle sandwich sounds a bit crazy, but yakisoba pan is incredibly cheap "street food" in Japan, and it is pure genius as far as I am concerned!
Mix together the soy sauce, oyster sauce, tomato ketchup, Henderson's relish, mirin, and MSG, then set aside.
Cook the udon noodles as per the instructions on the packet. I usually use wok-ready noodles. I soak them in boiling water for a couple of minutes, then separate them and refresh them in cold water.
Peel the garlic and chop it as finely as you can.
Slice the shiitake mushrooms into 3-4mm (⅛") thin slices.
Heat a wok over a very high heat, and when it begins to shimmer, add the oil. Throw in the shiitake mushrooms and store fry for 2 minutes.
Throw in the garlic and stir-fry for 30 seconds.
Add the drained noodles and stir fry for 1 minute.
Push the noodles to the side of the wok, add the sauce and then cook until the sauce reduces almost all the way down and coats the noodles nicely without there being a pool of sauce in the wok.
Remove from the heat, transfer to a bowl and allow to cool a little.
Slice the hotdog buns vertically but not all of the way through, then generously butter them on both sides of the cut.
Slice the spring onions at a 45° angle as thinly as you can.
Load the hotdog buns with the noodles, then top as desired with pickled ginger, spring onions, sesame seeds and mayonnaise. Special props if you use Japanese mayo!
Notes
I serve two buns per serving, because I am greedy!