Slice the beef: Use a sharp knife to slice the beef thinly against the grain. If needed, chill the meat in the freezer for 15–20 minutes first to make slicing easier.

Coat with cornstarch: Pat the beef dry with paper towels. Toss the slices with 3 tablespoons cornstarch until lightly coated. Shake off any excess. Let it rest 10 minutes while you prep the sauce.

Make the sauce: In a small bowl, whisk together soy sauce, brown sugar, and water or broth. Stir in rice vinegar and chili garlic sauce if using. In another small bowl, mix 1 teaspoon cornstarch with 1 tablespoon water to make a slurry. Set both aside.

Preheat your pan: Heat a large skillet or wok over medium-high until hot. Add 1 tablespoon oil.

Sear the beef in batches: Add a single layer of beef to the pan. Sear 1–2 minutes per side until browned but not fully cooked through. Transfer to a plate and repeat with remaining beef, adding more oil if needed. Do not crowd the pan.

Sauté aromatics: Lower heat to medium. Add a tiny splash of oil if the pan is dry. Add garlic and ginger. Stir 20–30 seconds until fragrant, but don’t let them burn.

Build the sauce: Pour in the soy mixture. Bring to a simmer for 1–2 minutes. Stir in the cornstarch slurry and cook until the sauce thickens and turns glossy.

Finish with beef and scallions: Return the seared beef and any juices to the pan. Toss to coat and simmer 1–2 minutes until the beef is just cooked and tender. Stir in the scallions and cook 30–60 seconds to keep them crisp.

Taste and adjust: Add a splash more vinegar for brightness, or a pinch of brown sugar for sweetness. If it’s too thick, loosen with a tablespoon of water.

Serve: Spoon over hot rice or noodles. Garnish with sesame seeds if you like.