Thinly slice beef: Hold your knife at a 45-degree angle to create wider, flatter slices. Slice the beef against the grain into thin pieces, about ⅛-inch thick.
Tenderize and marinate: In a bowl, add the beef and season with everything from soy sauce to sesame oil. Add the water little by little, massaging the beef until fully absorbed. Sprinkle in the starch and mix until evenly coated. Cover and marinate in the fridge for 15 minutes, or up to 1 day in advance.
Preheat the pan: Preheat a 12-inch (31 cm) sauté pan or wok over medium heat. Carefully place your hand about 2.5 to 3 inches (6 to 8 cm) above the pan surface to test the heat. When it feels too hot to keep your hand nearby, the pan is ready. Add the oil. It should shimmer but not smoke. If the pan gets too hot, set it aside for a few minutes to cool slightly.
Sear the first side: Turn heat up to medium-high. Add the beef and spread into a single layer. You should hear a strong sizzle—if not, increase the heat. Sear undisturbed for about 2 minutes, until browned on the bottom.
Sear the second side: Use a firm spatula to flip the beef and sear the other side for 30 seconds to 1 minute. The beef should be almost cooked through. Transfer the beef out, along with any pan juices.
Serve or turn it into a stir-fry: Turn off the heat. Transfer the beef and pan juices to a serving plate. Garnish with scallions and sesame seeds, if using. The beef is ready to eat as-is, or you can sauté aromatics and vegetables separately, then combine them with the beef to make your own tender beef stir-fry.
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Notes
Cookware: Use a large sauté pan or wok. A 12-inch (31 cm) pan works best because the beef has more room to sear instead of steam.
Use a splatter guard, not a lid: A lid traps moisture and can make the beef watery. A splatter guard helps with oil splatter while letting steam escape.
Best beef cuts: Skirt, flank, sirloin tip, flap, or sirloin steak all work well. These cuts are easier to slice thin and stay tender with quick cooking.
Knife tip: A sharp boning knife works well for thin, controlled slicing.
Slicing technique: Always slice against the grain at a 45-degree angle for wider, thinner pieces and more tender bites.
Heat matters: The pan should be hot enough to keep a steady sizzle once the beef goes in. If there’s no sizzle, the beef will release moisture and steam instead of sear.
Make ahead: You can slice and marinate the raw beef up to 1 day in advance. Keep it covered in the fridge until ready to cook.
Freeze: Add the sliced and marinated raw beef to a freezer-friendly bag and press it flat. Freeze for up to 2 months. Defrost overnight in the refrigerator before cooking.