Marinate the Beef: In a small bowl, combine soy sauce, sesame oil, brown sugar, Mirin, rice wine vinegar, minced garlic, and oyster sauce.
Add ribeye strips and marinate for at least 15 minutes.
Prep the Veggies: Slice the red bell pepper, chop the baby bok choy, and cut the green onions into 2-inch pieces.
Heat the Skillet: Heat a large nonstick skillet or wok over medium-high heat. Add avocado oil and heat until it shimmers.
Cook the Beef: Add marinated beef in a single layer. Cook undisturbed for 2-3 minutes until crispy. Flip and cook another 2-3 minutes until browned and tender. Remove and set aside.
Stir Fry the Veggies: In the same skillet, add more oil if needed. Stir fry the bell pepper and baby bok choy for 3-4 minutes until tender but crisp. Add green onions and cook for another minute.
Combine and Simmer: Return beef to the skillet. Pour in chicken or beef broth to deglaze the pan. Simmer for 2-3 minutes. Sprinkle with sesame seeds and stir.
Serve: Serve over cooked brown rice or white rice or rice noodles.
Optional: Add kimchi on the side and gochujang (thinned with some sesame oil) for extra spice.