Coat the bottom of a large pot with 2 tablespoons of oil and place it over medium heat.
When the oil is hot, add the mushrooms in an even layer and cook them for about 5 minutes, until browned on the bottoms. Flip and cook for about 5 minutes more.
Remove the mushrooms from the pot and transfer them to a plate.
Add the remaining tablespoon of oil to the pot, and give it a minute to heat up.
Add the onion, carrot, celery and garlic. Cook for about 10 minutes, flipping occasionally with a spatula, just until the veggies begin to soften and onions become translucent.
Return the mushrooms to the pot and add seitan, along with any juice in the package.
Sprinkle in the flour, a bit at a time, stirring frequently to form a thick paste that coats the veggies.
Stir in the wine and bring it to a simmer. The liquid should thicken up very quickly. Allow it to cook for about a minute, stirring frequently.
Stir in the broth, milk, nutritional yeast, soy sauce, rosemary, thyme and sage. Raise the heat and bring the mixture to a simmer.
Lower the heat and allow the mixture to cook at a low simmer for about 15 minutes, stirring occasionally, until the veggies are tender and the sauce has thickened up a bit.
Stir in the peas.
Remove the pot from heat, taste test and season with salt and pepper to taste. Adjust any other seasonings as desired.
Preheat the oven to 375° and gather a large (deep 10-inch diameter or larger) pie dish or a 2 quart casserole dish.
Use a rolling pin to roll the pie crust so that it's large enough to fully cover the top of the dish.
Transfer the vegetable stew into the dish, then drape crust overtop, trimming any low hanging edges. Cut a few slits in the top of dough.
Bake until crust is golden brown and the filling is bubbly, about 40 minutes.
If you don't like seitan, leave it out and double the mushrooms. If you don't like mushrooms, leave them out (skip steps 1-3 of the recipe) and double the seitan.