In the bottom of the slow cooker, layer the onions, carrot, celery and garlic cloves. Place the chicken breasts on top of the vegetables and pour the broth over the chicken. Sprinkle the chicken with thyme, salt and pepper. Place the lid on the slow cooker and cook for 3 hours on high (or 6 hours on low).
Remove the chicken from the slow cooker and shred into bite-sized pieces with a fork. Return to the slow cooker.
In a medium bowl, whisk the half and half together with the cornstarch. Pour the mixture into the slow cooker and stir to combine.
In a small bowl, combine the flour, baking powder, garlic powder and salt. Work in the butter using your fingers until course crumbs begin to form.
Stir in the milk and combine well until a sticky dough begins to form.
Drop tablespoon-sized dollops of dough over the stew in an even layer. Place the lid on the slow cooker and cook for 1 additional hour or until the dumplings are cooked through.