Trim any excess fat from the beef. Freeze the steak for 20–30 minutes to firm (this makes slicing easier). Using a sharp knife, slice the beef thinly across the grain into 1/8–1/4-inch strips.

Make the marinade

In a bowl combine soy sauce (3 tbs), rice vinegar (1 tbs), vegetable oil (2 tsp), granulated sugar (1 tsp), minced garlic (4 cloves reserved for cooking; 2 cloves go into marinade), grated ginger (1 tsp), and black pepper (1 tsp). Stir until sugar dissolves.

Marinate the beef

Place the sliced beef into the bowl with the marinade and toss to coat thoroughly. Cover and refrigerate for 20–30 minutes (or up to 2 hours). This seasons the beef and starts flavor infusion.

For Stir-frying

Prep vegetables and slurry

While the beef marinates, thinly slice the onion, red bell pepper, and green onions. In a small cup whisk together cornstarch (1 tsp) and water (2 tbs) into a smooth slurry and set aside.

Cook aromatics

Heat a large skillet or wok over high heat until very hot. Add neutral oil (2 tbs). When shimmering, add the remaining minced garlic (4 cloves) and grate of fresh ginger if desired; stir constantly for 15–20 seconds until fragrant but not browned.

Sear the beef

Increase heat to high. Add the marinated beef in a single layer—work in two batches if necessary so pieces sear instead of steam. Spread beef out and let it sear without moving for 30–45 seconds, then stir-fry for another 30–60 seconds until mostly cooked through but still juicy. Remove beef to a plate and keep warm.

Stir-fry the vegetables

Add the sliced onion and red bell pepper to the hot pan and stir-fry for 1–2 minutes until they begin to soften but remain crisp. Add the scallions and cook 30 seconds more.

Finish the sauce

Return the beef and any accumulated juices to the pan. Stir to combine. Give the cornstarch slurry a quick stir, then pour into the pan along with sesame oil (1 tsp) and kosher salt (1 tsp). Toss continuously for 30–60 seconds until sauce thickens and glazes the beef and vegetables. Taste and adjust seasoning: add more soy sauce or salt if needed.

Serve

Remove from heat. Transfer to a serving plate and sprinkle with toasted sesame seeds (optional). Serve immediately over steamed rice, noodles, or with a side of steamed vegetables.