In a small bowl, combine sliced flank steak with all marinade ingredients (1 tablespoon hoisin sauce, 1 teaspoon sriracha sauce, and 2 teaspoons cornstarch). Let sit at room temperature for 15-30 minutes (or refrigerate for up to 4 hours).
Whisk together sauce ingredients in a mixing bowl and set aside. Gather all stir fry ingredients and set aside. The key to a great stir fry is preparation!
Heat 1 tablespoon vegetable oil in large skillet over MED-HIGH heat. Cook beef in a single layer, 1-2 minutes, turning halfway through. If you need to, cook the beef in batches to avoid overcrowding the pan. Transfer cooked beef to a plate.
Add 1 tablespoon vegetable oil to same skillet and add broccoli florets. Cook broccoli 1 minute, then add water and cover the skillet to steam the broccoli for approximately 2-3 minutes. Remove broccoli to a plate.
If needed, add a small drizzle of vegetable oil to the same skillet, then add garlic and ginger. Cook 30 seconds, being careful not to burn the garlic. Add cooked beef and broccoli to the skillet and stir to combine.
Whisk sauce ingredients again to make sure they're combined, then add them to the skillet. Cook everything 1-2 minutes, stirring nearly constantly, until sauce is thickened to your liking, and all beef and broccoli is coated.
Remove from heat and sprinkle with green onions and sesame seeds if desired and serve. Can be served as is, or over white rice.
(Nutrition information provided is an estimate and will vary based on cooking methods and brands of ingredients used.)