Heat a medium skillet over medium–high heat on the stove top until hot. Add the butter, onion and garlic to the pan.
Saute onion and garlic in butter for about 3 minutes until the onion becomes translucent.
Add flour to the pan and cook and stir until the butter has mostly absorbed into the flour. Add mixture to a 5 to 6 quart slow cooker.
Add milk and chicken stock to slow cooker and stir well to combine.
Add chicken pieces, peas, carrots, poultry seasoning, salt and pepper to slow cooker and stir.
Cover and cook on LOW for 6 hours.
Unroll refrigerated pie crust and place it on top of the chicken mixture in slow cooker in a single layer. If you are using an oval shaped slow cooker you may want to slightly stretch the pie crust dough out to fit the slow cooker better.
Place a clean kitchen towel or layer of paper towels between the lid of the slow cooker and crock.
Continue to cook for 1 to 1 1/2 hours on LOW until the pie crust is cooked through.