Begin by trimming any excess fat from the boneless beef short ribs or chuck roast and cut the meat into 2-inch chunks. Pat the pieces dry to ensure a good sear. Chop one large yellow onion and set it aside.

In a large skillet, heat 1 tablespoon of olive oil over medium-high heat until shimmering. Add the beef chunks in batches, searing them for 2-3 minutes per side until nicely browned. Transfer the beef and any rendered juices to the slow cooker, layering it with chopped onions.

In a mixing bowl, whisk together 1 cup of low-sodium soy sauce, ½ cup of light brown sugar, ¼ cup of rice vinegar, and ¼ cup of toasted sesame oil. Add in 2 tablespoons of grated fresh ginger, 6-8 cloves of minced garlic, and optional ingredients like 1 tablespoon of gochujang and 1 tablespoon of honey. Mix until sugar dissolves completely.

Pour the prepared sauce over the seared beef and onions in the slow cooker. Stir gently to ensure even coating. Set the slow cooker to low and cook for 6-8 hours or high for 3-4 hours, until the beef is tender.

Once cooking time is up, carefully remove the beef from the slow cooker and shred it using two forks. If you prefer a thicker sauce, create a cornstarch slurry by mixing 1-2 tablespoons of cornstarch with 2-3 tablespoons of cold water and stir this into the sauce in the slow cooker. Let it cook for an additional 15 minutes until thickened.

Return the shredded beef to the slow cooker, stirring to coat the meat in the sauce. Taste and adjust seasoning as needed. Garnish with thinly sliced green onions and toasted sesame seeds before serving.