Cook the noodles according to the package instructions. Drain and set aside. Heat the vegetable oil in a large pan or wok over medium heat. Add the minced garlic and sliced onion, and stir-fry for about 1-2 minutes until fragrant and the onion turns translucent.
Add the sliced mixed vegetables to the pan and stir-fry for another 2-3 minutes until they are slightly tender but still crisp. Push the vegetables to one side of the pan and add the cooked noodles to the other side. This helps to keep the noodles from clumping together.
In a small bowl, whisk together the soy sauce, oyster sauce, dark soy sauce (if using), sugar, and sesame oil. Pour the sauce over the noodles and vegetables, and toss everything together until well combined.
Continue stir-frying for another 2-3 minutes, making sure the sauce is evenly distributed and the noodles are heated through. Remove from heat and taste the noodles. Adjust the seasoning if needed by adding more soy sauce, oyster sauce, or sugar according to your preference.
Transfer the Chinese noodles to serving plates or bowls. Garnish with sliced green onions, sesame seeds, and chopped cilantro if desired.
Serve hot and enjoy the savory and flavorful Chinese noodles!