Prepare all ingredients.
Soak fresh chow mein noodles in boiling water for 20–30 seconds. If using dried noodles, cook according to package instructions. Rinse with cold water and drain well.
Slice scallions, onion, and shallot as instructed. Keep scallion whites and greens separate.
In a bowl, mix all sauce ingredients until well combined.
Heat a pan over medium-high heat and add 1 tablespoon of oil.
Add scallion whites, onions, and shallots. Sauté until lightly caramelized, then push to one side.
Add another tablespoon of oil and the noodles. Stir-fry for 1–2 minutes until slightly dry.
Pour the sauce around the edges of the pan and toss everything together. Continue stir-frying for 1–2 minutes.
Add bean sprouts and scallion greens. Stir-fry briefly until just softened but still crunchy.
Taste and adjust seasoning if needed. Serve immediately for the best Cantonese Soy Sauce Chow Mein experience.