Build the sauce and add beef: Whisk together 1/2 cups coconut aminos, 1/3 cups beef broth, 3 tablespoons brown sugar, packed, 1 tablespoons sesame oil, 4 garlic cloves, minced, and 1 teaspoons fresh ginger, grated in the bottom of the crockpot. Add sliced flank steak, thinly sliced against the grain and toss to coat.
Slow cook the beef: Cook on LOW for 4–5 hours or HIGH for 2–3 hours, until beef is very tender.
Thicken sauce and add broccoli: Whisk 3 tablespoons cornstarch and 3 tablespoons cold water together until smooth. Stir into the crockpot to thicken the sauce. Add 4 cups fresh broccoli florets on top. Replace lid and cook on HIGH for 25–30 more minutes until broccoli is bright green and just tender.
Serve: Stir gently and serve over rice or noodles. Top with 3 green onions, sliced (for serving), 1 tablespoons sesame seeds (for serving), and 1/2 teaspoons red pepper flakes (optional) if using.