Preheat oven to 375 degrees.
On a lightly floured surface, carefully roll out one of the pie crusts. Transfer the pie crust to a prepared pie pan that has been sprayed with non-stick spray.
**Note - To help with the transfer of the dough, roll the dough loosely onto the rolling pin and unroll it carefully over the pie pan. Press the dough into the pie pan and allow the excess to fold over the sides of the pan. Set aside.
In a large stockpot over medium heat, melt the butter. Stir in the flour and cook for 2-3 minutes. Switch to a whisk and add the chicken stock, salt, pepper, and garlic. Whisk continually until the mixture heats and thickens (approximately 5-7 minutes).
Using a wooden spoon, stir in the thyme, mustard, pork, minced garlic, and frozen vegetables. Cook, stirring occasionally to heat through.
Carefully pour the pork filling into the prepared pie crust. On a lightly floured surface, roll the second pie crust out like the first one. Transfer the dough over the pork filling (on the rolling pin) and carefully unroll to cover.
Use a paring knife to trim the excess dough away (saving some for the leaf garnish), and use a fork to press and seal the bottom and top crusts together.
Use a paring knife to cut some steam holes into the top crust, and insert a pie bird (optional).
Use some of the trimmed dough to cut out leaf shapes and use a little water to adhere the leaves to the top of the pie (optional).
In a small bowl, mix the egg and 1 tablespoon water. Brush the egg wash over the top crust.
Place the pie on a baking sheet and bake in a preheated oven for 40-45 minutes or until the crust is golden brown and the filling is heated through.
Cool for at least 20 minutes and serve.