PREHEAT OVEN: Place oven rack to lower 1/3. Set oven to 425°F.
PREPARE BOTTOM CRUST: Using roll-out refrigerated pie crust, unroll crust into lightly greased 9-inch to 10-inch deep-dish pie plate. Place pie plate with crust into the freezer to temper until ready to fill.
PREPARE FILLING: Heat olive oil and butter in a large saucepan over medium heat. Add the potatoes, carrots, celery, mushrooms, frozen pearl onions, chives, parsley, sage, thyme, fresh ground black pepper, rosemary, bay leaf, and garlic. Cover, then saute, stirring every 3 minutes until vegetables are just firm-tender (do not overcook).
THICKEN: Sprinkle 5 tablespoons flour over saute mixture, then stir. Add the chicken stock, frozen peas, heavy cream, paprika, Worcestershire sauce, black pepper, turmeric, and nutmeg. Bring to a boil then reduce heat to medium-low. Remove bay leaf. Season fresh ground black pepper and 1 pinch celery salt (or to taste).
ADD CHICKEN: Fold in 2 cups boneless, skinless chunked rotisserie chicken.
FILL SHELL: Pour the prepared filling into frozen bottom shell. Place top crust dough over pie. Pinch and flute edges of upper crust to adjoin with bottom crust.
PREPARE GLAZE: In small bowl with one beaten egg, add 1 1/2 tablespoons water and whisk. Brush top crust and edges of pie lightly and evenly with mixture. Cut slits around top center for steam to escape.
BAKE pie at 425° about 30 minutes on lower rack, or until evenly golden brown.
COOL and set for at least 20 minutes before cutting and serving.
SLICE, serve and enjoy!