Begin by preparing your steak. Using a sharp knife, cut the flank steak across the grain into thin slices. Toss with corn starch and allow to rest for 10 minutes.
Heat oil in skillet over medium-high heat. Add beef slices; stir-fry for 2–3 minutes until browned, doing so in batches.
Remove beef, and in the same skillet, cook garlic and ginger for about 30 seconds.
Add soy sauce, hoisin sauce, and brown sugar to skillet; stir until bubbled.
Return beef to skillet, toss in sauce, and cook for another 2–3 minutes.
Notes
Store any leftovers in an airtight container in the fridge for up to three days, reheating as needed.