Make pastry: In a food processor, pulse flour, thyme, salt and pepper. Add butter and shortening; pulse until mixture resembles coarse sand. With motor running, add 6 Tbsp. ice water and mix until dough comes together, adding more water if necessary. Turn dough out onto floured surface and knead just to bring together. Shape into a rectangle, wrap in plastic and chill.
Make filling: Place chicken breasts in a large pan and cover with broth. Bring to a boil over medium heat, then reduce heat to low and simmer until chicken is cooked through, about 20 minutes. Remove chicken from broth and cut into chunks. Strain broth; reserve.
Melt butter in large skillet over low heat, add onion and celery and cook until softened, about 15 minutes. Stir in salt, pepper, flour and thyme and cook, stirring, 5 minutes. Add reserved broth, increase heat to medium and bring to boil, whisking constantly. Reduce heat and simmer until thickened, whisking often, about 3 minutes. Stir in wine and cream and heat through. Add chicken, vegetables, potatoes and parsley. Season with salt and pepper.
Mist a 9-by-13-inch baking dish with cooking spray. Pour in chicken mixture. Allow to cool slightly. Preheat oven to 375°F. Line a baking sheet with aluminum foil.
Roll out dough to 1/4-inch thickness between 2 sheets of plastic wrap or waxed paper. Roll crust onto rolling pin, then transfer it on top of baking dish, covering filling. Trim crust to fit, crimping edges. Knead scraps together, roll out and use Halloween cookie cutters to create shapes for top of pie. Brush top of pie with beaten egg, gently press on cut-outs and brush them with glaze. Cut a few slits in crust to vent steam. Place on lined baking sheet and bake until crust is golden and filling is bubbling, about 60 minutes. Serve hot