Mix sauce ingredients in a small pan on medium heat. Stir until gently simmering. Dissolve cornflour with equal parts water in a separate cup, then stir into the sauce mixture. Stir until thickened, set aside.
Season diced chicken breast with salt and pepper. Heat a large wok or deep skillet over medium-high heat and spray with olive oil cooking spray.
Add seasoned chicken pieces and cook for 6-8 minutes, stirring occasionally, until golden brown and cooked through. Add minced garlic to hot pan and stir-fry for 30 seconds until fragrant. Remove from pan and set aside.
In the same pan, re-spray if needed. Add julienned carrots and sliced celery. Stir-fry for 2-3 minutes.
Add mung bean sprouts to the vegetables and stir-fry for another minute.
Add udon noodles to the pan. Add ¼ cup of water to loosen them up. Gently separate them if they're stuck together. Stir-fry for 2-3 minutes until they're heated through.
Return cooked chicken to the pan.
Pour sauce over the noodle mixture.
Toss everything together and cook for 2-3 minutes, until evenly coated.
Taste and season with additional salt and pepper if needed.
