Place the cubed chicken in the slow cooker, then add the onion, celery, carrots, mushroom and garlic.
In a bowl, combine the chicken soup, chicken broth, and heavy cream. Stir the cornstarch and water together and stir into the soup, then add the celery salt, poultry seasoning, and ground mustard.
Stir the soup mixture into the meat and cook on high for 2 hours or low for 3 hours.
Thirty minutes before the chicken is finished cooking, stir in the green peas and pimentos. Continue cooking for the remaining 30 minutes.
For the puff pastry:
Heat the oven to 350° and spray a cookie sheet with cooking spray.
Unroll the puff pastry, then cut into 8 pieces. Beat the egg and water and brush over the pieces. Place on the prepared pan and bake for 7 minutes, or until golden brown.
Serve the chicken a la king over the puff pastry squares.
Notes
Place any remaining soup into an airtight container and keep it in the fridge for up to 4 days. Reheat the leftovers over medium to medium high heat until warmed throughout.
If you don't want to use puff pastry, mashed potatoes, sauteed vegetables, and white rice can be served with this dish. You can also sprinkle fresh parsley on top.
Green bell peppers, bell pepper, or green beans are vegetables you can add in addition to the other vegetables.