Prep the sauce: In a bowl, whisk orange zest, orange juice, soy sauce, rice vinegar, sugar or honey, sesame oil, garlic, ginger, and red pepper flakes.
In a separate small cup, mix 1 teaspoon cornstarch with 1 tablespoon water to make a slurry. Set both aside.
Boil the noodles: Cook noodles according to package directions until just shy of al dente. Drain and toss with a teaspoon of oil to prevent sticking.
Keep nearby.
Season and coat the chicken: Pat chicken dry. Toss with salt, pepper, and egg white if using. Add 1/2 cup cornstarch and toss until the pieces are evenly coated and powdery.
Crisp the chicken: Heat a large skillet or wok over medium-high.
Add 2 tablespoons oil. Cook chicken in two batches without crowding, turning occasionally, until golden and cooked through, about 6–8 minutes per batch. Transfer to a plate.
Add remaining 1 tablespoon oil as needed.
Stir-fry the vegetables: In the same pan, add broccoli and a splash of water. Stir-fry 2 minutes. Add bell pepper, carrot, and the white parts of the green onions.
Cook 2–3 minutes more, until crisp-tender.
Thicken the sauce: Pour the orange sauce into the pan with the vegetables. Let it bubble. Stir in the cornstarch slurry and cook 30–60 seconds until glossy and slightly thick.
Add chicken and noodles: Return the chicken to the pan and toss to coat.
Add noodles and the green onion tops. Tangle everything together with tongs until the sauce clings to the noodles.
Taste and adjust: For more brightness, add a squeeze of orange or a splash of rice vinegar. For more salt, add a dash of soy sauce.
For more heat, sprinkle extra red pepper flakes.
Finish and serve: Top with sesame seeds and a little extra orange zest. Serve hot.