In a saucepan over medium heat, combine mirin, soy sauce, and sugar, and bring to a light boil for one minute to caramelize sauce.
Spray slow cooker insert with cooking spray.
Place chicken in the bottom of the slow cooker and top with vegetables.
Pour sauce over chicken and ensure it's evenly coated.
Cook on low for 3.5 hours.
In a small bowl, whisk together cornstarch and cold water. Add to slow cooker and stir.
Cook for another 15 minutes to thicken sauce.
If desired, shred chicken.