Preheat oven to 350 degrees F.
To a large mixing bowl, add vital wheat gluten, onion powder, coriander, allspice, sea salt, add black pepper. Mix well to combine.
Add 1 1/2 cups water, tamari, 1 teaspoon of coarse brown mustard, and apple cider vinegar. Stir to blend then knead 2 minutes, until all ingredients are well combined and texture is smooth and springy. Shape into a loaf and set aside.
To a small mixing bowl, add remaining coarse brown mustard and light brown sugar. Mix well then spread in an even layer over seitan. Press pickling spice blend over the top.
Place roast in a prepared 9" x 13" baking dish. Pour vegan beef broth around roast (not over the top!). Cover tightly with foil and bake 1 hour. Remove foil cover and continue baking an additional 30 minutes.