a 14-inch ovenproof skillet; a 3-inch round biscuit cutter
To make the filling: In a large (14-inch) ovenproof skillet, melt the butter and olive oil over medium heat. Add the bell pepper, onion, carrot, and garlic and saute until soft.
Stir in the peas, corn and thyme. Sprinkle the flour over the vegetables and then add the chicken broth and cream. Stir to distribute the flour and the liquids. Simmer until the sauce thickens, about 10 minutes. Stir in the turkey and add salt and pepper to taste. Set the mixture aside to cool slightly.
To make the biscuit top crust: In a large mixing bowl, combine the flour, salt, sugar, and baking powder. Whisk for thirty seconds to combine and evenly distribute the leavening. Add the shortening and butter and toss to coat the fat a bit with the flour. Using your fingertips, gently work the fats into the flour until the mixture resembles coarse cornmeal with some pea-size chunks of fat.
In a small bowl, whisk together the buttermilk and egg, and add this to the flour mixture. Stir together with a wooden spoon. Then, gently work through the mixture with your hands to separate any wet patches and bring the dough together.
Turn the dough out onto a floured work surface, sprinkle with the minced thyme and knead gently until it just holds together. Pat it into a rough 1/2-inch-tall disk and cut it into 7 biscuits using a 3-inch round biscuit cutter. (Bake off the scraps and use for croutons in the Broccoli Salad recipe.)
To assemble: Preheat the oven to 400 degrees F. Top the filling in the skillet with the biscuit rounds. Brush the tops with egg wash and sprinkle them with sea salt.
Bake the potpie for 25 to 30 minutes, until the filling is bubbling and the biscuits are golden brown.