Heat the oil in a large wok or pan. Add the onions and garlic and sauté for 1 to 2 minutes, just until the vegetables are soft and fragrant.
Add the sliced chicken and cook just until it's cooked, about 4 to 6 minutes. Add the shrimp. Then add the soy sauce and oyster sauce. Stir.
Add the carrots, celery, cabbages, snap peas, and mushrooms. Stir well to combine. Add the broth and cook for 2 to 3 minutes.
Add the noodles and use a wooden spoon to push them down, tossing gently to give the noodles equal coverage in the liquid. Cook until the liquid is absorbed, about 8 to 10 minutes.
Be sure to stir the ingredients constantly throughout this time, so that they don't start to stick to the bottom.
Serve and garnish with green onions, if desired.
Video
Notes
Make ahead: You can make this up to 24 hours before serving. Allow to cool completely, then store in an airtight container in the refrigerator. Before serving, reheat in the wok or pan with a teaspoon of vegetable oil and a few drops of soy sauce to help "wake up" the noodles. Reheat until the noodles, vegetables, and meat are heated through. Storage: Store leftovers in an airtight container in the refrigerator for up to 4 days.