Directions

1. Prepare the crust. Pulse 1 cup flour, baking powder, 1/4 teaspoon salt in a food processor until combined. Add the butter, one piece at a time, pulsing until the mixture looks like coarse meal.
   
2. Separate the egg; refrigerate the egg white. Beat the egg yolk and 2 tablespoons milk in a bowl, then add to the food processor, pulsing a few times. Add ice water and pulse until the dough comes together. Turn out onto a lightly floured surface and gather into a ball. Flatten into a disk, wrap in plastic wrap and chill at least 1 hour.
   
3. Meanwhile, make the filling. Prick the potatoes with a fork and microwave on high for 8 minutes until tender. Cool slightly, then peel and break into small pieces. Preheat the oven to 425 degrees.
   
4. Heat the olive oil in a large pot over medium heat. Add onion and cook until soft, about 4 minutes. Add carrots, celery, thyme, and 1 tablespoon butter, then season with salt, pepper and garlic powder and cook for another 4 minutes.
   
5. Sprinkle in 3 tablespoons flour and stir until lightly toasted, about 3 minutes. Add 1/3 cup milk, potato pieces and broth and simmer until thickened, about 15 minutes. Remove from the heat and stir in the chicken, yogurt, peas and parsley. Season with 2 teaspoon salt, 1/4 teaspoon pepper, and 1/2 teaspoon garlic powder.
   
6. Transfer the filling to a 2-quart casserole dish. Roll out the dough on a lightly floured surface until about 1/8 inch thick and slightly larger than the dish. Press the dough against the sides of the dish. Beat the reserved egg white in a bowl; brush over the dough and season with salt, pepper, and garlic powder. Sprinkle the Parmesan all over the top and place on a baking sheet. Cut 3 slits at the top and bake until the crust is golden brown, 20 to 25 minutes.