01 - Place sliced beef in a large bowl. Toss with cornstarch and black pepper until evenly coated.
02 - In a separate bowl, whisk together soy sauce, beef broth, brown sugar, oyster sauce, garlic, ginger, sesame oil, and red pepper flakes until well combined.
03 - Place coated beef into the crock pot. Pour sauce over the beef and stir to combine thoroughly.
04 - Cover and cook on low for 4 hours, or on high for 2 hours, until beef is tender.
05 - About 30 minutes before serving, stir in broccoli florets. Cover and cook until broccoli is just tender but still bright green, about 25-30 minutes.
06 - Serve hot over steamed rice. Garnish with sesame seeds and green onions if desired.