Preheat oven to 425 degrees. Spray pie dish or cast iron skillet with nonstick spray.
In a large skillet, spray nonstick spray and add onion, garlic, and chopped mushrooms; sauté oven medium heat for 2 minutes. Add all frozen vegetables; increase heat to medium high and sauté for 5 minutes. Add cream of celery soup and milk, stir to combine. Add salt, pepper, thyme, rosemary, and tarragon and stir; sauté for 5 more minutes. Add cheese and mix to melt.
Unroll 1 pie crust and place onto bottom of baking dish. Pour vegetable mixture into the pie crust. Unroll the second crust and then place on top, pinching the sides together to close. Cut slits in the top of the crust.
Place into oven and bake for 20-25 minutes, checking at 20 minutes. Remove from oven when top crust is browning.
Place butter on top of crust, brush around the crust. Cut into slices and serve.
This pot pie reheats deliciously!
Can be frozen - make sure to completely cook after baking. Cover completely and freeze. To cook, remove from freezer and allow to defrost for 30 minutes, place into oven, covered at 350 degrees for 30 minutes. Then remove foil, cook an additional 50-60 minutes or until heated through. Alternatively, you could defrost all day, then cook in oven at 350 for 40 minutes.