Preheat Oven: Start off by preheating your oven to 375°F (190°C). This will ensure it's ready when you are.

Prepare Vegetables: In a large skillet, melt the butter over medium heat. Add the onion and garlic, and sauté until they're tender and fragrant.

Make the Roux: Stir in the flour to the onions and garlic, and cook for about 1 minute, stirring constantly, until a thick paste forms.

Create Creamy Base: Gradually add the chicken broth to the skillet, whisking as you go to eliminate lumps. Then, add the milk and continue to cook, stirring frequently, until the mixture is thick and bubbly. This should take about 3-5 minutes.

Season the Mixture: Add salt, pepper, thyme, and rosemary to the creamy base. Stir to combine.

Combine Ingredients: Add the chopped turkey and frozen mixed vegetables to the skillet. Mix everything thoroughly so that the turkey and veggies are evenly distributed in the creamy sauce.

Prepare Pie Crust: Fit one pie crust into the bottom of a 9-inch pie pan. Pour the turkey and vegetable mixture into the crust.

Top and Seal: Place the second pie crust over the top. Crimp the edges to seal, and make a few small slits in the top to allow steam to escape.

Egg Wash: Brush the top crust with beaten egg to achieve a golden-brown finish.

Bake: Place the pot pie in the preheated oven and bake for 40-45 minutes, or until the crust is golden brown and the filling is bubbly.

Cool and Serve: Let the pie cool for about 10 minutes before slicing and serving.