Marinate the chopped chicken in the sauce and set aside while preparing the other ingredients.
Heat a skillet or pan over medium-high heat. Add a thin layer of oil.
Add the chicken with the marinade and cook for about 5 minutes, stirring occasionally, until roughly 60% cooked.
Meanwhile, boil the udon noodles according to package instructions. Drain and set aside.
Add the broccoli florets and red bell pepper to the skillet. Stir-fry until the chicken is nearly fully cooked.
Add the cooked noodles and toss everything together until the chicken is fully cooked and coated in sauce.
Garnish with toasted sesame seeds and serve immediately.