Slice the flank steak thinly against the grain and toss with the cornstarch in a bowl; let it sit 10 minutes. In a small bowl, whisk the soy sauce, water, honey and sesame oil.
Heat the avocado oil in a large skillet or wok over high heat. Add the beef in a single layer and sear 1-2 minutes per side until browned; transfer to a plate (cook in two batches if needed).
Add the broccoli and 2 tbsp water to the hot skillet, cover and steam 3 minutes. Uncover and stir-fry 2-3 minutes until crisp-tender with a few charred spots.
Stir in the garlic, ginger and red pepper flakes; cook 30 seconds until fragrant.
Return the beef and any juices to the pan, pour in the sauce and toss 1-2 minutes until glossy and the beef reaches 145 °F.
Serve hot over steamed rice or noodles, spooning the extra sauce over the top.