Mix the sauce: In a small bowl, whisk together soy sauce, oyster sauce, sesame oil, brown sugar, and water. Set aside.
Cook the noodles: Bring a pot of water to the boil and cook the egg noodles according to packet instructions. Drain and rinse briefly under cold water to stop the cooking. Set aside.
Cook the prawns: Heat 1 tablespoon of oil in a wok over high heat. Add the prawns and stir-fry for 1–2 minutes, tossing constantly, until just pink and opaque. Remove and set aside.
Sauté aromatics: Add the remaining oil to the wok, followed by the garlic, ginger, and white parts of the spring onion. Stir-fry for about 30 seconds, just until fragrant.
Add noodles and sauce: Add the cooked noodles and pour over the sauce. Toss well for 1 minute to coat evenly and heat through.
Combine and finish: Return the prawns to the wok along with the snow peas. Toss for about 30 seconds, just until the snow peas turn bright green and everything is glossy.
Serve: Divide among bowls and top with green spring onions, toasted sesame seeds, a drizzle of sesame oil, and optional chilli oil or flakes if you’d like a little heat.