In a medium mixing bowl whisk together the soy sauce, oyster sauce, ginger and sugar. Whisk until sugar is completely dissolved.
Whisk together the chicken broth and corn starch / flour and set aside
Place the pork in a ziplock back, cover with marinade, press out the air from the bag, seal and massage to coat the meat. Roll the bag up tight and place in the fridge at least 10 mins.
In the mean time, chop your veggies. Chop the stalks of the green onions into 2" long pieces and the stalks of the bok choy into 3/4" - 1" pieces and set aside together.
Chop the leaves of both the bok choy and the onions into about 1 1/2" - 2" pieces- they're gonna shrink drastically in the pan so you still want big mouthfuls of them!
Set a pot of water to boil, but don't add the udon noodles yet.
In a large, deep pan or wok heat vegetable oil on high. Add the meat with all its marinade and stir fry til most pieces are mostly brown (1-3 mins). It's fine if they're still a little pink in the center, they'll finish cooking in the sauce and come out just right. Remove the meat and set aside, leaving the sauce.
Add the bok choy and onion stalks with the garlic to the pan and stir fry til they soften (about 30-60 seconds).
Return the meat to the pan along with vegetable leaves and chicken broth. Toss thoroughly.
Add the udon noodles to the boiling water and break apart with fork, cooking to just before package directions, strain then add to the pan / wok and toss some more to thoroughly coat with sauce. Then serve!
Optionally drizzle in sesame oil and white rice vinegar and toss some more right before serving.