Melt the butter in a large heavy bottomed pot, then add in the onion, carrots and celery. Season with salt and pepper, then cook, stirring occasionally until the onions turn translucent, about 5 minutes. Add in the garlic, and cook for 2 minutes more. Sprinkle the flour over top, then stir until everything is evenly coated and all the flour is moistened. Pour in the chicken stock and apple cider while stirring, and cook for about 10 minutes until the mixture has thickened. Stir in the heavy cream and thyme, then remove from the heat. Taste and adjust the seasoning as needed. Stir in the chicken, peas and parsley, then allow to cool completely until proceeding with the recipe. Pouring a hot filling into the crust will make the dough difficult to work with and will not bake up as well.