Thinly slice flank steak with a sharp knife.
Generously salt and pepper sliced steak.
Mix sauce ingredients in a small bowl.
In a large pan or wok on medium high heat, heat oil until shimmery.
Add steak to pan, stirring occasionally to prevent burning.
When steak has started to brown, after about 5 minutes, add broccoli.
Keep heat up and keep stir frying beef and broccoli until broccoli is slightly soft and beef is lovely golden brown.
Add sauce to pan, bring sauce to boil, stirring to coat beef and broccoli evenly.
Reduce heat and let sauce thicken, about 2 minutes.
Remove from heat and serve, topping with sesame seeds.