Cook the chow mein or egg noodles according to package instructions. Drain and set aside.
Heat the vegetable oil in a large skillet or wok over medium-high heat.
Add the sliced onion and minced garlic. Sauté for 2-3 minutes until fragrant and the onions are translucent.
Add the sliced bell pepper, julienned carrot, and snap peas or green beans. Stir-fry for about 3-4 minutes until the vegetables are crisp-tender.
If using cooked chicken, beef, or shrimp, add it to the skillet and stir to combine with the vegetables.
Add the cooked noodles to the skillet and toss to combine with the vegetables and protein.
Stir in the soy sauce, oyster sauce (if using), hoisin sauce (if using), and sesame oil. Mix well to coat the noodles evenly.
Season with ground black pepper to taste.
Cook for an additional 2-3 minutes, stirring frequently, until everything is heated through and well combined.
Garnish with chopped green onions and fresh cilantro.
Enjoy your meal!