If starting with uncooked chicken, place the chicken in an instant pot or slow cooker with at least 1 cup of broth or water (if using instant pot).
Cook the chicken for 8 minutes/lb (instant pot) or 4-6 hrs on high/8-10 hours on low (slow cooker).
Once chicken is cooked, remove from the slow cooker to cool. Chop or shred and set aside.
If using the chicken broth, strain into a glass measuring cup and set aside.
If making a butter crust, take the butter out of the fridge to soften.
Preheat the oven to 400 degrees.
Using a veggie chopper or chef knife, finely chop the onions, carrots and celery and place into large pot or Dutch oven.
Add the butter or ghee and saute over medium-high heat for ~5-10 minutes until the veggies are softened and translucent.
While the veggies are sauteing, combine the chicken broth and milk in a 4 cup glass bowl or measuring cup and microwave for approximately 4 minutes to warm (this step isn’t necessary, but helps the mixture to boil and thicken more quickly).
Once the veggies are soft, add the gluten-free flour and stir over medium heat for approximately 2 minutes to create a roux.
Add the chicken broth and milk (or warmed mixture) to the veggie roux. Reduce heat to medium, stirring almost constantly until the mixture begins to thicken and bubble. Stir for 1-2 minutes until fully thickened.
Stir in shredded chicken.
Drain and rinse peas and potatoes (dice potatoes if necessary) and stir into the pot-pie mixture.
Add salt and pepper to taste.
If using a Dutch oven, set mixture aside. If using pie plates or other sized pans, pour the mixture into the pan(s) and set aside. Note: this mixture will fill 2 pie plates or 1-9x13 pan.