In a large mixing bowl, whisk together flour, baking powder, sugar, and salt. Set aside.

Cut your stick of butter into small cubes. Place all pieces into the bowl with the flour mixture. Using a pastry blender, cut cold butter into flour mixture until it is roughly the size of peas. Pour in cold buttermilk. Stir with a rubber spatula or wooden spoon until a shaggy dough forms.

Turn dough (and any unincorporated flour bits) out onto a floured surface. Flour your fingertips and pat the dough into a ½-inch thick rectangle. Fold dough in half, and turn one quarter turn. Pat out until it is ½-inch thick again. Repeat folding/quarter-turning/patting out until you have done it four times total. Re-flour your surface as necessary.

Slice dough in half and wrap each piece in plastic wrap. Chill while you prepare the filling.

Melt 2 tablespoons of butter in a large skillet over medium heat. Add mushrooms and a pinch of salt. Cook, stirring frequently, until browned and dramatically smaller. Set aside.

Add remaining 2 tablespoons of butter to the pan and swirl to coat. Add carrots, celery, onion, garlic, diced potato and ¼ teaspoon of salt. Cook, stirring frequently, for about 10 minutes. Add water and cover pan with a lid (or a sheet of foil) for another 5 minutes. Vegetables are done when you can easily stab a piece of celery with a fork.

Add mushrooms back to the pan. Sprinkle on flour, ¼ teaspoon salt, and pepper. Stir for about 2 minutes. Turn heat down to medium-low. Add mustard, thyme, rosemary, parsley, chicken stock and cream. Cook, stirring frequently, for 8-10 minutes, or until sauce thickens. Remove from heat. Stir in chicken and peas. Set filling aside while you roll out the crust.

Preheat oven to 400F. Lightly grease a pie plate.

Flour a surface and a rolling pin. Unwrap one half of the biscuit dough. Use rolling pin to roll dough to ⅛-inch thickness (about 14 inches in diameter for a 9-inch pie plate). For easiest rolling, roll dough in one direction, turning it one quarter-turn after each roll. Re-flour surface and rolling pin as needed.

To transfer to a pie plate, carefully fold dough into quarters. Place point in the center of the pie plate and carefully unfold. Fit it to the pan and trim any excess overhang (I didn’t have any). Fill with filling.

Repeat rolling process with the second half of the dough. Drape it over the filling, trim any excess overhang, and twist or crimp the edges as desired. Use a small knife to cut a few vents in a decorative pattern.

Make the egg wash. Combine egg and water in a small bowl. Whisk with a fork. Brush over all exposed crust.

Bake chicken pot pie for 30-32 minutes, until crust is deeply browned and filling is bubbling. Let cool 15 minutes before serving. Chicken pot pie will not slice cleanly.

Leftovers will keep covered in the refrigerator for up to four days. Leftovers will slice cleanly, as sauce thickens during cooling.