Place the fresh chow mein noodles in a large heatproof bowl.
Pour boiling water over the noodles and allow them to soak for about 30–60 seconds, or just until they loosen and soften slightly.
Drain thoroughly. Shake away as much excess water as possible and set the noodles aside.
Prepare the stir-fry sauce by combining the dark soy sauce, regular soy sauce, oyster sauce, fish sauce, sugar, and water in a small bowl.
Stir until the sugar has dissolved, then set the sauce aside.
Heat 1 tablespoon neutral oil in a large wok over medium-high to high heat.
Add the sliced yellow onion and white portions of the green onions.
Stir-fry for approximately 1 minute, until fragrant and beginning to soften. Transfer the aromatics to a plate.
Return the wok to medium-high heat and add another tablespoon of oil.
Add the drained noodles and spread them into a relatively thin, even layer.
Leave the noodles mostly undisturbed for about 2 minutes so the bottom can develop golden, crispy areas.
Carefully turn or flip sections of noodles. Drizzle the remaining tablespoon of neutral oil around the edges of the wok.
Continue pan-frying for another 2 minutes, or until both sides have developed lightly crisp, golden sections.
Return the cooked onions and green onion whites to the wok.
Add the bean sprouts and green portions of the green onions.
Pour the prepared sauce evenly over the Cantonese Chow Mein.
Using tongs or chopsticks, gently toss everything together for 1–2 minutes. Avoid stirring too aggressively, which can break the noodles.
Remove the wok from the heat once the sauce evenly coats the noodles and the bean sprouts remain slightly crisp.
Drizzle with sesame oil and toss once more.
Serve immediately while the noodles are hot and retain their crisp-chewy texture.