First, make the stuffing. Heat the oil in a medium skillet over medium-high heat, then add the onion and a pinch of salt and cook until softened and translucent, about 10 minutes. Add the mushrooms and garlic, and cook for an additional 3 to 5 minutes, until the mushrooms have released their liquid and the garlic is fragrant. Pour in the white wine or apple juice and allow it to bubble and reduce for a minute or so, scraping up any flavorful bits from the bottom of the pan. Add the salt, cooked farro, cranberries, pecans, fresh herbs, and a few grinds of black pepper, and mix well. Taste, adjust seasonings as necessary, and set aside until needed. (The stuffing can be made well in advance and refrigerated until needed.)
To make the seitan, preheat the oven to 350°F, and prepare your work surface by laying out two large (at least 20 inches long) pieces of foil, overlapping by a few inches. This is where you’ll roll up the seitan and stuffing.
Next, place the tofu, vegetable broth, oil, nutritional yeast, tamari, onion powder, garlic powder, poultry seasoning, salt, and pepper in the bowl of a food processor or a blender, then process until smooth. Pour into a medium-large bowl. Add the wheat gluten, and use a wooden spoon to stir the mixture together. Do not be alarmed when the mixture pulls together into a ball of dense dough with little effort: this it the wheat gluten pulling everything together. Use your hands to gently mix the dough right in the bowl until it looks uniform, then transfer the dough to the foil sheets. Use your hands to gently pat the dough into a rectangle roughly 16 by 10 inches. If a hole appears in the dough, no worries: just pinch it together with your fingers and continue.
When the dough is as close as you can get it to a rectangle, spread the mushroom and farro stuffing over the surface of the dough, leaving a 1-inch border around the edge. Press the stuffing gently and evenly into the dough, then starting at the edge closest to you and using the foil to help you, begin to roll the dough away from you, sushi-style, until you’re left with a log of stuffed seitan. Roll up the log snugly in the foil, twist each end together like a Tootsie Roll, then place on a baking sheet and transfer to the preheated oven.
Bake the seitan for 50 to 60 minutes, until it feels very firm when squeezed. (Use an oven mitt or kitchen towel to protect your hand when checking.) Remove from the oven, allow to cool for 10 minutes, then remove the foil. To serve, slice gently with a serrated knife, arrange slices on a serving platter, and serve hot with vegan gravy.