Step 1: In a medium bowl, combine chicken pieces with 1 tablespoon soy sauce, 1 teaspoon sesame oil, and 1 teaspoon cornstarch. Toss to coat and set aside to marinate for at least 10 minutes. Meanwhile, cook lo mein noodles according to package directions until al dente. Drain, rinse briefly with cold water, and toss with a tiny drizzle of sesame oil to prevent sticking.
Step 2: In a small bowl, whisk together the remaining 3 tablespoons soy sauce, 2 tablespoons oyster sauce, 1 tablespoon brown sugar, 1 teaspoon rice vinegar, ½ teaspoon ground ginger, ½ teaspoon garlic powder, and the remaining 1 tablespoon cornstarch with 3 tablespoons water or chicken broth until smooth.
Step 3: Heat 1 tablespoon vegetable oil in a large wok or skillet over high heat until shimmering. Add the marinated chicken in a single layer (do not overcrowd). Cook for 3-4 minutes, stirring occasionally, until golden brown and cooked through. Remove chicken from the pan and set aside.
Step 4: Add another ½ tablespoon vegetable oil to the same wok/skillet. Add sliced bell pepper, shredded carrots, and sliced onion. Stir-fry for 3-5 minutes until vegetables are tender-crisp.
Step 5: Return the cooked chicken to the wok with the vegetables. Add the cooked and drained lo mein noodles. Pour the prepared sauce mixture over everything. Toss continuously for 2-3 minutes, allowing the sauce to thicken and coat all ingredients. Stir in the chopped green onions. Taste and adjust seasoning if necessary. Serve immediately.