Preheat the oven to 425 F. Press 1 pie crust in a 9-inch pie plate, and trim to fit. Set aside.
Melt the butter in a large skillet over medium heat. Once melted, cook the onion, celery, and carrots until very soft, about 10 minutes.
Then, lower the heat to medium-low, cook the garlic for 1 minute, and stir in the flour. Mix continuously until it becomes thick like a paste.
Once the flour is completely incorporated, add the broth and mushroom soup. If you are using leftover green bean casserole, add in 3 cups. Stir continuously until thickened, and add extra stock if needed.
Stir in the rosemary, thyme, salt, pepper, and turkey. If applicable, add the can of trimmed green beans. Stir to warm through.
Pour or scoop the mixture into the prepared pie crust. Top with the second pie crust, and flute or crimp the edges to seal. With a small knife, cut slits into the top to allow steam to escape while cooking.
You can optionally cover the edges of the pie with tin foil. For a crispier crust, you can mix 1 egg yolk and 3 tablespoons of water together, and brush it onto the top crust.
Bake until pot pie is golden brown, about 20 to 25 minutes. Remove from the oven, and let cool for 5 minutes. Slice, and serve warm.