Melt the vegan butter or olive oil in a small stockpot. Add the onion and garlic and sauté for 5-8 minutes on medium heat until the onions are translucent and fragrant.
Add the mushrooms, seitan, wine, and bouillon. Cook the mushrooms for about 15 minutes, or until tender.
Add the flour, mustard, and Bragg Liquid Aminos, if using. Add the water if it looks like it needs a little liquid. Stir in the sour cream, tarragon, salt, and pepper and heat through.
Serve over pasta, mashed potatoes, roasted potato wedges, or rice!