Combine the mirin, soy sauce, water, sugar, cornstarch, ginger, and crushed red pepper flakes in a small saucepan over medium-low heat. Cook until thickened, about 8 minutes.
Season the chicken with salt and pepper.
Preheat your griddle to medium heat and oil it well.
Add the chicken and veggies and drizzle with 3 tablespoons of the sauce.
Cook for 8 minutes, stirring occasionally, or until the chicken is cooked through.
Drizzle with the sesame oil and stir well. Remove from heat. Serve with the remaining sauce.
