In a bowl, combine the vital wheat gluten, nutritional yeast, onion powder, and salt. Stir to mix well.
Add 1 tablespoon of the soy sauce, 1 tablespoon of the oil, and the water, and stir until well mixed.
Turn the mixture out onto a lightly floured work surface and divide it in half. Stretch one of the pieces into a flat rectangle, then place it between two sheets of plastic wrap or parchment paper. Let the dough rest for 5 minutes, then use a rolling pin to flatten it as much as you can (it will be elastic and resistant).
Repeat with the remaining piece. Top both layered dough rectangles with a baking sheet weighed down with canned goods for 10 minutes and set aside.
In a large skillet, heat the remaining 1 tablespoon of oil over medium heat. Add the seitan and cook until golden brown on both sides, turning once, about 4 minutes per side.
Pour enough water into the skillet to cover the seitan and add the remaining 2 tablespoons soy sauce. Bring just to a boil, then reduce heat to low and simmer, uncovered, for 45 minutes.
Remove the seitan from the water and set aside to cool.
Duxelles
In a large skillet, heat 1 tablespoon of the oil over medium heat. Add the onion, cover, and cook until softened, 5 minutes.
Stir in the mushrooms, spinach, soy sauce, wine, parsley, thyme, sage, salt, and pepper. Cook, stirring, until the spinach is wilted and the mushrooms release their juice, about 4 minutes.
Continue cooking until the liquid from the mushrooms evaporates and the duxelle mixture is nearly dry. Remove from heat and set aside to cool.
Place the pastry sheet on a lightly floured surface and divide into thirds, reserving one third for another use. Roll out the remaining two pieces of the pastry with a floured rolling pin.
Place one of the seitan cutlets in the center of each piece of pastry. Spoon a mound of the duxelle mixture in the center of each piece of seitan and roll up the seitan to enclose the stuffing. Then gently wrap the pastry around the seitan to enclose it. Moisten the edges with water, if needed, to help seal the pastry around the seitan.
Refrigerate for 30 minutes or until ready to serve. If not serving right away, cover and refrigerate for up to 24 hours.
When you're ready to bake your roulades, preheat the oven to 400°F. Place the pastry-wrapped roulades on a baking sheet, seam-side down and bake until the pastry is golden brown, about 25 minutes. Serve hot.