Step 1: If searing the beef (recommended for richer flavor), pat beef cubes dry with paper towels. Season generously with salt and pepper. Heat olive oil in a large skillet over medium-high heat. Sear beef in batches until browned on all sides. Transfer seared beef to your slow cooker.
Step 2: In a medium bowl, whisk together soy sauce, beef broth, brown sugar, sesame oil, grated ginger, minced garlic, and red pepper flakes (if using). Pour this mixture over the beef in the slow cooker.
Step 3: Cover and cook on low for 6-8 hours, or on high for 3-4 hours, or until the beef is fork-tender.
Step 4: About 30 minutes before serving, prepare the cornstarch slurry: In a small bowl, whisk together cornstarch and cold water until smooth. Stir the slurry into the slow cooker with the beef and sauce. Cook, uncovered, for 15 minutes to allow the sauce to thicken.
Step 5: Stir in the fresh broccoli florets. Cover and continue to cook for another 15-20 minutes, or until the broccoli is vibrant green and tender-crisp.
Step 6: Taste and adjust seasoning if necessary. Serve hot over cooked white rice, garnished with sesame seeds and sliced green onions if desired.