Put all creamy sauce ingredients into a blender and blend until nice and smooth. Reserve one tablespoon mixed with the ¼ cup milk on the side to brush pie if you choose. It is not needed, just will give an extra browning on the top.
Sauté onions in oil (or broth) and salt until nice and caramelized. About 5-7 minutes.
Add mushrooms, carrots and celery and sauté until mushrooms have released their liquid and it has mostly evaporated. About 5 minutes.
Add peas and thyme. Cook 2-3 minutes.
Add creamy sauce and cook until thickened. About 2-3 minutes.
If haven’t already, put raw pie crust into pie dish by rolling over the rolling pin and slowly transferring to pie dish. See post for photos.
Put filling into raw pie crust. Top with second raw pie crust and cut slits in top. See post for photos.
If you choose, brush with milk/creamy liquid.
Bake for 45 minutes until nice and golden.
If you are not using a high speed blender, you can soak the cashews overnight, boil them for 30 minutes or grind the dry cashews into a fine powder using a coffee grinder (do not soak them for this method).
If you are subbing broth or water for the milk, the end result may not be as creamy but will still have creaminess to it.