In the bottom of the slow cooker, layer the onions, celery and garlic cloves. Place the chicken breasts on top of the vegetables and pour the stock over the chicken. Sprinkle the chicken with thyme, salt and pepper.
Remove the chicken from the slow cooker and shred into bite-sized pieces with a fork. Return to the slow cooker. Stir in the peas and carrots.
Whisk the half and half together with the cornstarch. Pour the mixture into the slow cooker and stir to combine. Cook for an additional 20 minutes.
If using pie crust, while the soup is still cooking, roll out a refrigerated pie crust on a baking sheet coated with parchment paper and bake according to package instructions. Cool to room temperature and break up into bite sized pieces.
Ladle the soup into bowls and top with the pie crust crumbles. Sprinkle with parsley and fresh thyme, if desired. Serve immediately.
(Nutrition information provided is an estimate and will vary based on cooking methods and brands of ingredients used.)