Prep the beef: Freeze the steak for 15 minutes to firm it up. Slice thinly against the grain. Toss with cornstarch until lightly coated and no dry patches remain. Set aside.

Make the sauce: In a small bowl, whisk soy sauce, brown sugar, and water or broth. Add rice vinegar and chili flakes if using. Keep this near the stove.

Heat the pan: Use a large skillet or wok over medium-high heat. Add 1–2 tablespoons oil and let it shimmer.

Sear in batches: Add half the beef in a single layer. Cook 1–2 minutes per side until browned with a slight crust. Remove to a plate. Repeat with remaining beef, adding oil if needed.

Sauté aromatics: Reduce heat to medium. Add a drizzle of oil. Stir in garlic and ginger for 20–30 seconds until fragrant. Don’t let them burn.

Add veggies: Toss in bell pepper and the white/light green parts of the green onions. Stir fry 1–2 minutes until crisp-tender.

Bring it together: Return beef to the pan. Pour in the sauce. Stir and let it bubble for 1–2 minutes until glossy and slightly thickened. If too thin, simmer another minute. If too thick, splash in a tablespoon of water.

Finish: Stir in sesame oil and the dark green onion tops. Taste and adjust—add a pinch of sugar for sweetness, soy for salt, or vinegar for brightness.

Serve: Spoon over hot rice. Sprinkle with sesame seeds. Enjoy immediately while the beef is tender and the sauce is silky.