Step 1: Gather and prep all your ingredients. Thinly slice the chicken, chop vegetables, mince garlic and ginger. In a medium bowl, whisk together the soy sauce, oyster sauce, brown sugar, rice vinegar, sesame oil, and white pepper for the sauce. Set aside.
Step 2: Bring a large pot of salted water to a rolling boil. Add the spaghetti and cook for 1-2 minutes less than package directions, aiming for al dente. Drain, rinse under cold water to stop cooking, and toss with a tiny bit of sesame oil to prevent sticking.
Step 3: Heat 1 tablespoon of vegetable oil in a large wok or very large skillet over medium-high heat. Add the chicken pieces in a single layer and cook for 3-4 minutes until nicely browned and almost cooked through. Remove chicken and set aside, leaving drippings in the pan.
Step 4: Add another tablespoon of oil to the wok if needed. Add minced garlic and ginger, stir-fry for 30 seconds until fragrant. Immediately add sliced onions and carrots. Stir-fry for 2-3 minutes until onions soften.
Step 5: Add sliced bell peppers and cabbage to the wok. Continue to stir-fry for another 2-3 minutes until vegetables are vibrant and crisp-tender.
Step 6: Return the cooked chicken to the wok with the vegetables. Add the cooked spaghetti. Pour the prepared lo mein sauce evenly over everything. Using tongs, toss constantly and vigorously for 2-3 minutes until all ingredients are thoroughly coated in the sauce and heated through.
Step 7: Remove from heat. Garnish generously with chopped green onions and sesame seeds, if desired. Serve your homemade chicken spaghetti lo mein hot and enjoy immediately.