How to Make It
Preheat the oven to 425°. Put the chicken in a medium roasting pan, season generously with salt and pepper. Roast the chicken for 20 minutes, until it is partially cooked. Discard the skin and shred the meat. Leave the oven on.
Meanwhile, in a medium saucepan, boil the stock until reduced to 2 cups, about 15 minutes; keep warm.
Melt the butter in a large, deep skillet. Add the mushrooms, season with salt and pepper and cook over high heat, stirring occasionally, until softened and golden, about 6 minutes. Sprinkle the flour over the mushrooms and cook, stirring, until evenly coated. Add the reduced stock and the cream and whisk until thickened, about 3 minutes. Add the shredded chicken, parsley and thyme and season the filling with salt and pepper; let cool.
In a food processor, pulse the flour with the salt. Add the butter and pulse until the mixture resembles coarse meal. Add the sour cream and pulse just until the pastry comes together. Turn the pastry out onto a lightly floured surface and divide it into thirds; pat two of the thirds into 1 disk and the remaining third into another disk. Wrap the disks in plastic and refrigerate them until firm, at least 30 minutes.
On a lightly floured surface, roll out the larger piece of pastry to a 15-inch round. Transfer the pastry to a deep 10-inch glass pie plate. Spoon the chicken filling into the pastry and brush the rim with the egg wash. Roll out the remaining pastry to an 11-inch round and carefully lay it on top of the filling. Press the edges of the pastry to seal and trim the overhang to 1 inch. Fold the overhanging pastry dough under itself and crimp it decoratively to seal. Cut 4 slashes in the top crust and brush with the egg wash.
Bake the pie for 20 minutes, or until the pastry is lightly golden. Lower the temperature to 350ï¿½ and bake for 40 minutes longer, or until the top and bottom crusts are golden and the filling is bubbling. Cover the pie loosely with foil if the top crust browns too quickly. Let the chicken pie stand for 15 minutes before serving.