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

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

Working in batches, sear the beef cubes on all sides until deeply browned. Don’t crowd the pan. Remove browned beef to a plate.

Reduce heat to medium. Add the chopped onion to the pot and cook, stirring occasionally, until softened (about 5–7 minutes).

Add the minced garlic and grated ginger. Cook for 1 minute until fragrant.

Return the seared beef to the pot. Pour in the soy sauce, beef broth, brown sugar, rice vinegar, and red pepper flakes if using. Stir to combine and make sure the beef is mostly submerged.

Bring the pot to a simmer. Reduce heat to low, cover tightly, and braise gently for 2–3 hours, or until the beef is fork-tender and starting to fall apart. Check every 30–45 minutes and add a splash of beef broth if the liquid is getting low.

Once the beef is tender, remove the lid. In a small bowl, whisk cornstarch and cold water until smooth to make a slurry. Slowly pour the slurry into the simmering sauce while stirring constantly.

Continue to simmer and stir for 2–3 minutes until the sauce has thickened to a glossy glaze that coats the back of a spoon.

Spoon extra sauce over the beef and serve immediately with rice or noodles.