Cook the noodles. Boil according to package directions until just al dente. Drain, rinse briefly under cool water to stop the cooking, and toss with a little oil to prevent sticking.
Set aside.
Make the sauce. In a small bowl, whisk soy sauces, oyster sauce, hoisin (if using), sesame oil, sugar, cornstarch, water, and a pinch of black pepper. Taste and adjust—the sauce should be savory with a slight sweetness.
Prep the chicken and veggies. Slice chicken thinly against the grain. Cut vegetables into bite-size pieces.
Keep aromatics (garlic, ginger, green onion whites) separate from the greens for later.
Heat the pan. Place a large skillet or wok over medium-high heat. Add 1 tablespoon oil. When shimmering, add chicken in a single layer.
Stir-fry the chicken. Cook 3–4 minutes, stirring occasionally, until just cooked through and slightly browned.
Remove to a plate.
Sauté aromatics. Add remaining 1 tablespoon oil to the pan. Add garlic, ginger, and green onion whites. Stir 30 seconds until fragrant—don’t let them burn.
Cook the vegetables. Add the mixed veggies.
Stir-fry 2–3 minutes until crisp-tender. Harder veggies like carrots or broccoli may need another minute; keep them bright and snappy.
Combine everything. Return the chicken and any juices to the pan. Add the cooked noodles and toss to mix evenly.
Add the sauce. Pour the sauce around the edges of the pan and toss constantly for 1–2 minutes.
The sauce should thicken and coat the noodles. If it looks dry, splash in another tablespoon of water.
Finish and serve. Stir in green onion greens. Taste and adjust salt, pepper, or heat.
Serve hot with sesame seeds or chili oil if you like.