Place beef in a stockpot and pour in enough water to cover; bring to a boil. Remove from heat and drain.
Pour vegetable oil into a slow cooker; add beef, 8 cups water, beef broth, soy sauce, rice wine, green onions, brown sugar, garlic, chile peppers, chile paste, ginger, star anise, and Chinese five-spice powder.
Cook on Low for 8 to 9 hours.
Transfer beef to a bowl with a slotted spoon. Strain broth into a bowl. Discard solids.
Bring a pot of water to a boil; add bok choy for 30 seconds. Remove and run under cold water. Cook udon noodles in the boiling water until tender yet firm to the bite, about 4 minutes. Drain.
Divide noodles among serving bowls and top with beef, baby bok choy, and broth. Top with Chinese pickled mustard greens.