Spray a 6-quart slow cooker with nonstick spray. Add the diced onion. Top with the chicken pieces in an even layer.
In a medium bowl, stir together the soups, thyme, garlic powder and black pepper. Pour the soup mixture over the chicken. Pour in the chicken broth.
Cook on HIGH for 4 hours. Stir in the thawed frozen vegetables.
On a lightly floured surface, gently roll out each biscuit into an oval shape, about 1/4 inch thickness. Cut each oval into 6 even pieces.
Carefully arrange the biscuit pieces over the top of the chicken and vegetables. Cover and cook for 45-60 minutes longer, until the dumplings are cooked through.
Transfer the chicken to a serving platter using a slotted spoon. Use two forks to shred the chicken, then return it to the slow cooker. Gently stir the mixture to combine, allowing some dumplings to break up into the broth.
Serve the chicken and dumplings warm. Garnish with chopped parsley, if desired.
