Combine cornstarch and beef broth, stir to mix well. Set aside.
Season the beef with salt and pepper. Toss coat, let them sit for at least 10 minutes.
After 10 minutes, cook the beef. Heat a pan with oil, add the beef, cook for 3-4 minutes on each side or until well seared and browned. Remove from the pan. Set aside.
Use the same pan, add more oil if needed. Add garlic and onion into the pan, saute until tender or aromatic. Add the shiitake mushrooms, saute until tender and browned.
Add celery, carrot, and bell pepper, stir to combine. Cover the pan for a minute or until the veggies are tender.
Remove the cover, add soy sauce and oyster sauce, constantly stirring until well combined. Add the cabbage, cooked beef, and slurry, stir, simmer for a minute or until the sauce thickened. Serve warm.