Direction

1. Place beef in a stockpot and cover with water; bring to a boil. Remove from the heat and drain.
2. Pour vegetable oil into a slow cooker; add beef, 8 cups water, broth, green onions, soy sauce, rice wine, brown sugar, garlic, chile peppers, chile paste, ginger, star anise, and five-spice powder. Cover and cook on Low for 8 to 9 hours.
3. Transfer beef to a bowl with a slotted spoon. Strain broth into a bowl; discard solids. Return broth to the slow cooker; keep on Warm until needed.
4. Bring a pot of water to a boil; add bok choy and cook for 30 seconds. Remove with a slotted spoon and run under cold water. Drain, then roughly chop.
5. Add udon noodles to boiling water; cook until tender yet firm to the bite, about 4 minutes. Drain.
6. Divide noodles among serving bowls and top with beef, bok choy, and hot broth. Garnish with mustard greens.