Start by cutting the beef into cubes. Season with salt and pepper to taste. Heat olive oil in a large pot or Dutch oven over medium heat, then add the beef and sear until browned on all sides (about 5-7 minutes). Once the beef is browned, remove it from the pot and set aside.
In the same pot, add the sliced onion and cook for 3-4 minutes until softened. Add the garlic and grated ginger, cooking for another minute until fragrant.
Return the browned beef to the pot and stir in the soy sauce and sugar. Let it cook for 2 minutes to allow the flavors to combine.
Pour in the beef broth, stirring to combine. Bring the mixture to a simmer, then cover and cook on low heat for 45-60 minutes, or until the beef becomes tender.
Once the beef is tender, stir in the pickled mustard greens. Cook for an additional 5-7 minutes until the greens are heated through and have absorbed the flavors of the beef.
Taste and adjust the seasoning if necessary, adding more soy sauce, salt, or pepper. Serve the beef and pickled mustard greens hot over rice or noodles, and enjoy!