Season bite-sized chuck roast pieces generously with salt and pepper. Add 1 tbsp of olive oil to a large, high-walled skillet over high heat. When the oil is very hot and shimmering, add HALF of the beef.
Brown the beef for about 2-4 minutes until the beef pieces have good color. You’re not looking to cook the beef, only brown it. Remove the beef to a plate once it has browned and then repeat the same steps for the second half of the beef (adding another tablespoon of oil to the skillet).
Place all of the browned beef into the slow cooker and now prepare the sauce.
In a medium bowl, whisk together the beef broth, soy sauce, sesame oil, mirin, Gochujang, brown sugar, ground ginger, garlic cloves, and sliced green onions until well combined. Pour the sauce over the beef and cook on high for 3-4 hours until the beef is tender and the sauce has thickened slightly.
To thicken the sauce more, add the cornstarch slurry, stir, and cook on high for another 20-30 minutes or until it has thickened to your liking.
Serve the beef and sauce over rice or noodles. Top with sliced green onions and toasted sesame seeds. Enjoy!!