Par-cook and dry the noodles: Bring a large pot of water to a boil. Add the noodles and cook just until they loosen and turn barely tender — about 1 minute for fresh noodles, or 1 minute less than the package direction for dried. Drain, rinse briefly under cold water, shake off as much water as you can, and spread on a rimmed tray in a loose layer to dry for 10 to 15 minutes, tossing once or twice.
Mix the supreme soy sauce: Stir the light soy sauce, dark soy sauce, sugar, Shaoxing wine, water or stock, and white pepper together in a small bowl until the sugar dissolves. Set near the stove. Hold the toasted sesame oil for the end.
Sear the noodles: Heat a wok or large cast-iron skillet over high heat until it just begins to smoke. Add 2 tablespoons of the oil and swirl to coat. Add the dried noodles in an even layer, spread out, and press lightly. Leave undisturbed for 2 minutes, until the bottom turns golden and crisp in spots. Flip in sections and cook 1 to 2 minutes more. Move the noodles to a plate.
Stir-fry the vegetables fast: Return the wok to high heat. Add the remaining 1 tablespoon of oil. Add the onion (if using) and stir-fry 30 seconds. Add the bean sprouts and stir-fry 45 to 60 seconds, just until they lose their raw edge but still snap. Add the garlic chives and toss for 20 seconds, until wilted slightly and bright.
Bring it together: Return the noodles to the wok. Pour the soy sauce mixture around the edge of the hot wok so it sizzles on contact, then toss everything together quickly for 45 to 60 seconds, until the noodles turn glossy and evenly brown with no puddle of sauce in the pan. Take the wok off the heat and stir in the toasted sesame oil.
Serve right away: Pile the noodles onto a warm platter with some crisp strands on top. Scatter extra chives or scallions over the plate and serve at once, with chili oil at the table if you like heat.