Freeze the flank steak for 30–60 minutes to make slicing easier. Slice thinly against the grain and season lightly with coarse salt.
Whisk together the soy sauce, water, brown sugar, sambal oelek, rice vinegar, sesame oil, and cornstarch until smooth.
Cook the rice according to package directions.
Heat a large skillet over medium-high heat and add a little vegetable oil.
Cook the beef in batches without overcrowding the pan. Let each batch brown before turning. Remove and set aside.
Add another drizzle of oil and stir-fry the broccoli for 2–3 minutes until bright green. Add a splash of water if you prefer softer broccoli. Remove from the skillet.
Reduce the heat slightly. Add the remaining oil, then sauté the garlic and ginger for about 1–2 minutes until fragrant.
Pour in the prepared sauce and stir until it thickens into a glossy coating.
Return the beef and broccoli to the skillet and toss until evenly coated with the sauce. This Sesame Beef and Broccoli Recipe comes together quickly, so avoid overcooking the beef.
Serve over cooked rice and garnish with sliced green onions and sesame seeds.