Peel and dice 1 small onion and wash and slice 2 cups of mushrooms. Then add 2 tablespoons of oil to a large skillet and saute the diced onion, sliced mushrooms, and 1/4 tsp salt.
Add 2 tsp smoked paprika, 1 tsp oregano, 1/2 tsp black pepper, 1/2 tsp garlic powder, 1/4 cup soy sauce, and 2 tbsp tomato paste, and mix well. Then stir in 1 cup of water and turn off the heat.
Transfer the mixture to a food processor or blender and puree.
Pour the pureed mixture into a large glass bowl add 1 1/2 cup of vital wheat gluten and stir with a silicone spatula.
Kneed in the last 1/2 cup of wheat gluten with your hand until a firm dough ball is formed.
Shape your dough ball into a loaf shape and wrap it tightly in thick foil.
Place the wrapped roast in a steaming basket and steam for 1 hour and 15 minutes. (Or in a pressure cooker or Instant Pot for 55 minutes and then let the steam escape naturally.)
Baste and bake the faux beef
Preheat the oven to 375° F (190° C).
Let your loaf sit for about 5 minutes so you can safely open the foil without getting burnt by the steam inside. Then unwrap the steamed seitan and put it into a small baking dish that is just a little bigger than your loaf.
Mix together the basting liquid (oil, water, soy sauce, paprika, oregano, rosemary, and black pepper). Pour it evenly over your roast. Then use a spoon to scoop up the basting liquid and drizzle it over any spots that you may have missed.
Bake your plant-based roast beef for 30 minutes at 375° F (190° C), taking it out of the oven every 10 minutes to quickly spoon the basing liquid over the top of the roast.
Notes
Use a glass bowl and a silicone spatula to mix your seitan. Vital wheat gluten is very sticky once it gets wet and will stick like glue to a plastic bowl or a wooden spoon.
Wheat gluten is a very fine powder that will get everywhere if you are not careful with it. Pour it low down and slowly over the broth mixture so it doesn't splash out of your bowl.
Knead the seitan until all of the gluten flour is incorporated and you have a dough that's a little firmer than you would have for bread. Only knead it for about 60 seconds so your wheat meat doesn't get too chewy.
If you don't want the foil to touch your food, you can wrap the mock beef in parchment paper first, and then wrap the foil around that.
Seitan expands when it cooks, so be sure to use strong foil, or it will break the foil when it is steaming.