Prepare the vegetables per the instructions in the ingredients list and turn your slow cooker on (low for a 6-7 hour cook time or high for a 4-5 hour cook time).
Add the onions, carrots, celery, potatoes, garlic, and the sage to the slow cooker and stir to combine them.
Place the rosemary, thyme, and the bay leaf on top of the vegetables.
Season the chicken breasts with the salt and pepper and place them on top of the herbs and vegetables.
Pour the chicken broth into the slow cooker.
Cook the stew on the low-temperature setting for 6-7 hours or on the high-temperature setting for 4-5 hours. Make sure the chicken is cooked through and reaches a temperature of at least 165℉ and that the vegetables are tender and cooked through.
When there is about 30 minutes left of the cook time, remove the chicken breasts, rosemary, thyme, and bay leaf from the slow cooker. Discard the herbs. Shred the chicken into bite-sized pieces.
Before returning the chicken to the slow cooker, make the cornstarch slurry. Mix the cornstarch and cold water together until smooth and combined. Stir the slurry into the slow cooker and then add the chicken back in.
Let the stew cook for the remaining time and then serve warm, garnished with fresh herbs and salt and pepper to taste.