Pat the beef cubes dry with paper towels and season with salt and pepper. Heat the oil in a large pot or Dutch oven over medium-high heat. Add the beef in a single layer, working in batches if needed, and brown on all sides. Transfer the browned beef to a plate.
Reduce the heat to medium. Add the smashed garlic, sliced ginger, and the white parts of the green onions to the pot. Cook for 1 minute until fragrant. Stir in the Chinese five-spice powder and cook for another 30 seconds.
Pour in the sherry to deglaze the pot, scraping up any browned bits from the bottom. Return the beef and any accumulated juices to the pot.
Add the beef broth, star anise, and cinnamon stick. Bring the liquid to a boil, then reduce the heat to low, cover, and let it simmer for 2 hours, or until the beef is fork-tender.
Once the beef is tender, use a slotted spoon to remove it from the broth and set aside. Strain the broth through a fine-mesh sieve into a clean pot; discard the solids (ginger, garlic, spices).
Stir the light soy sauce, dark soy sauce, and oyster sauce into the strained broth. Bring to a gentle simmer and taste, adding more soy sauce if needed. Keep the broth warm.
Cook the noodles in a separate large pot of boiling water according to package directions. In the last 1-2 minutes of cooking, add the bok choy to the water to wilt it slightly. Drain the noodles and bok choy well.
To serve, divide the noodles and bok choy among four large bowls. Top with the reserved beef. Ladle the hot broth over everything. Garnish with the sliced green parts of the green onions.