Add the chopped onion and diced celery. Sauté for 3 minutes until soft. Then add the sliced mushrooms, minced garlic, dried thyme, and dried rosemary.
Add the boiled potatoes and carrots to the skillet, along with the frozen peas, corn, and chopped parsley. Stir to combine well. Remove the skillet from the heat.
Pour in the vegetable filling. Roll out the second crust and place it on top. Press the edges firmly to seal and cut a few slits in the top to allow steam to escape.