To prepare the crust, Combine flour, sugar, and salt in a large bowl; cut in butter and mix with fingertips until mixture resembles coarse meal. Add vinegar and water, tbsp at a time; stir just until moist. Press mixture gently into a 5-inch circle on heavy-duty plastic wrap; cover with additional plastic wrap. Chill for 15 minutes or overnight until needed.
To prepare the filling, bring 5 cups Chicken Stock to a boil in a medium saucepan. Add the potato and carrot; cook for approximately 4 minutes. Meanwhile, Melt the butter in a very large skillet over a medium heat and the onion; cook for 2-5 minutes until the onions just begin to sweat. Combine 1 cup flour, corn starch and 1 cup remaining Chicken Stock in a small bowl; whisk until well combined. Add to the skillet. Stir in the potato mixture with the reserved cooking liquid, turkey or chicken, peas, seasonings, half-n-half, sherry, and celery. Remove from heat (you can store for a day or two until you wish to bake it).
Pour approximately 1-2 cups filling into ramekins of choice (or you can pour the entire filling into a casserole dish to bake it whole. Metal bowls work too).Pre-heat the oven to 400 degrees farenheit.
Roll the dough out to about 1/4-1/2 inch-thick and cut the appropriate size for your pot pie Place the dough atop of the filling (either press the dough into sides of the ramekin, or just leave flat; feel free to be decorative) and cut four slits in the center of the dough (to let the steam escape). Whisk the egg with the 4 tbsp half-n-half and brush onto the dough and bake for approximately 40-65 minutes; until the crust is a golden brown and puffed.