Melt butter in a large pan. Cook onion, carrot, and celery until tender, about 3 minutes. Stir in flour and sage. Add turkey and continue to cook for another 3 minutes.
Pour in milk. Cook and stir until thick and bubbly. Season with salt and pepper. Add green peas and continue to cook for a couple of minutes. Remove and set aside to cool.
Combine all-purpose flour and butter in food processor. Pulse until mixture resembles coarse bread crumbs. Add cold water and pulse again until a sticky dough forms.
Turn dough onto a large sheet of plastic. Divide dough into two, one piece about 5/8 and the other 3/8 of the entire quantity dough. Pat to form two disks. Wrap and chill in the refrigerator for 30 minutes.
Putting it together
Prepare 4 individual 5-inch pie dishes.
Roll out the larger piece on a lightly floured surface to a size sufficient to line 4 individual pie dishes. Divide the dough into 4 and line pie dishes with dough. Trim off excess at the edge of each pie dish.
Divide turkey filling evenly between the 4 pie dishes.
Roll out the remaining piece of dough and divide into 4. Place a piece of dough on the top of each pie dish and trim off excess dough. Seal by pressing down the pastry onto the edge of each pie dish with the back of a fork. Cut a few slits on the top to let out steam.
Brush with egg wash and bake in a 350°F (180°C) oven for 45 to 50 minutes.