You'll find absolutely nothing lacking in this classic dish. It's creamy, full of flavor, super rich, and tastes just as good as the chicken pot pie your mom used to make. Perhaps, even better. 😉
Preheat oven to 400. In a large sauce pan over medium heat, melt butter. Add chopped onion and sauté until it begins to become opaque, about 3 minutes. Add cooked seitan and cook for a few more minutes. When the onions become translucent, add in the 3T of flour. Stir the chickun and onion into the flour until everything is well coated, and continue stirring as it will begin to start sticking.
Add one cup of broth, plus the thyme and parsley and bring mixture up to a simmer. Add in the plant milk, carrots, celery, and string beans or peas, and continue to stir until the mixture begins to thicken. Salt and pepper to taste. Keep stirring because as it thickens it will begin to stick. You want it to get very creamy, until the moisture has reduced by about 1/3.
If using pie crusts, fit one to the bottom of a large pie plate. If using puff pastry, I found that a square casserole dish works well, but a pie plate will also work. Spoon your mixture into the prepared dish. Top with either the other pie crust or puff pastry. Brush the dough with a little more soy milk to help it brown. Poke holes so there is room for steam to escape.
Bake for 30 minutes uncovered. Rest for about 20 minutes to allow it to firm up a little bit and serve.
TIP: Add a sheet pan or foil to the rack below your pot pie in case it bubbles over a bit.