Slice the beef sirloin into thin strips against the grain.
In a bowl, mix soy sauce, brown sugar, garlic, ginger, and sesame oil.
Place the sliced beef in the crockpot.
Pour the sauce mixture over the beef, stirring to coat.
Cook on low for 4-6 hours or high for 2-3 hours.
30 minutes before serving, add the broccoli florets to the pot.
In a small bowl, mix cornstarch and water until smooth.
Stir the cornstarch mixture into the crockpot to thicken the sauce.
Cook for an additional 30 minutes on high.
Serve hot, garnished with sesame seeds.