Pat the beef cubes dry with paper towels and season generously with salt and black pepper.

In a large Dutch oven or heavy-bottomed pot, heat vegetable oil over medium-high heat.

Sear the beef cubes on all sides until they develop a deep brown crust, then transfer to a plate.

Reduce heat to medium and add chopped onion to the pot. Cook until softened, about 5-7 minutes.

Add minced garlic and grated ginger, and stir for another minute until fragrant.

Return the seared beef to the pot and pour in soy sauce, beef broth, brown sugar, rice vinegar, and red pepper flakes; mix well.

Bring mixture to a gentle simmer, then lower heat to low. Cover and let braise for 2-3 hours.

Check occasionally to ensure there's enough liquid, adding more beef broth if necessary.

Once beef is tender, remove lid and prepare a cornstarch slurry by whisking together cornstarch and cold water.

Gradually add the slurry to the pot, stirring constantly until the sauce thickens, about 2-3 minutes.

Serve immediately with extra sauce drizzled over beef, paired with rice or noodles.