Melt butter in large sauce pan. Add the chicken pieces with salt and pepper and partially cook them, then add the onions, celery, and garlic. Saute until onions are translucent. Stir in evaporated milk, mushroom soup, and the other vegetables. Blend well. Pour into unbaked pie crust. Top with another unbaked crust. Seal edges. Cut slits in the center of top crust to vent steam. Bake at 350 degrees for 45 to 50 minutes.