Melt the butter in a large skillet over medium heat. Add the onions, carrots, and celery. Cook until softened, about 10 minutes. Stir in the flour and cook for 3 minutes.
Slowly pour in the chicken broth, stirring constantly. Add the chopped turkey, peas, mustard, and seasonings. Stir and bring to a gentle bubble.
Mix in the sour cream and ½ cup of heavy cream. Stir until bubbling again, then remove from heat
Preheat the oven to 400° Fahrenheit.
Roll out one pie crust slightly larger on a floured surface. Fit it into a 9-inch deep pie dish with the excess hanging over the edges.
Spoon the turkey filling into the prepared crust.
Place the second pie crust over the filling. Seal the edges by pressing them together, then flute with your fingers or press with a fork.
Brush the top crust with heavy cream. Cut several small slits in the top to allow steam to escape.
Bake for 25 to 30 minutes until the crust is golden brown. Let the pie rest for 20 minutes before slicing and serving.