Preheat the oven to 400°F.

Melt the butter in a medium pot over medium heat.

Add the onion, carrot and celery and cook until the onion is translucent, about 4 minutes.

Add the garlic and cook until fragrant, about 1 minute.

Add the mushrooms and cook until they have softened and released their liquid, about 5 minutes.

Add the flour and stir to coat the vegetables.

Cook until it is no longer white and coats the bottom of the pan, stirring constantly, about 1 minute.

Add the broth and stir to combine.

Bring to a boil, stirring occasionally, and simmer until the mixture thickens slightly, about 3 minutes.

Remove from the heat and stir in the chicken, cream, thyme, and salt. Set aside to cool slightly.

Meanwhile, line a 9-inch pie plate (not deep dish) with one pie crust.

Pour the filling into crust and top with the second pie crust.

Seal the edges with a fork or decorative crimp, and cut 3 or 4 small vents in the top of the crust.

Bake on the lowest rack of your own, placed on a rimmed baking sheet, until the crust is golden brown all over, 35 to 40 minutes. Cool slightly before slicing.