Whisk together soy sauce, oyster sauce, brown sugar, cornstarch, and water in a small bowl until smooth.

Pat beef dry, then slice thin against the grain. Trim snow peas, mince garlic, and grate ginger.

Heat vegetable oil in a large wok or skillet over high heat until shimmering.

Add beef in a single layer, cooking undisturbed for 1 minute until edges are golden, then stir-fry for another minute until just browned.

Push beef to one side, add garlic and ginger, stir constantly for 30 seconds.

Add snow peas and stir-fry for 1–2 minutes until bright green and crisp-tender.

Stir sauce mixture, pour over beef and peas, cook 1 minute, stirring constantly until sauce thickens.

Remove from heat, transfer to serving plates, and sprinkle with green onions and sesame seeds.