To a 5qt or 6qt slow cooker or larger, add chicken breasts, cream of chicken soup, onion, garlic, and chicken stock, and then season with dried parsley, salt, and pepper. Give everything a quick stir. Add the pieces of butter, cover, and cook on LOW for 3-4 hours, or on HIGH for 2-3 hours.
When the internal temperature of the chicken is 165°F/74°C, take it out and put it on a cutting board.
Add uncooked noodles and frozen vegetables to the crockpot. Stir so that the noodles are submerged.
Shred the chicken and return it to the crockpot. Cook, covered, on HIGH for 20-25 minutes, or until the noodles are cooked.
When the noodles are tender, stir again, have a taste and adjust seasonings to your preferences. Serve in bowls with fresh chopped parsley on top.