Add all dry ingredients to a large mixing bowl and roughly stir.
Combine all wet ingredients in a large container, then form a well in the dry ingredients and gently pour in about ⅓ of the liquid.
Using a spatula, gently combine the dry and wet ingredients. Keep pouring in more liquid until both are combined well and form a smooth & slightly sticky dough.
Remove the dough from the bowl and knead it with your hands on a floured surface for about 2 minutes.
Place the dough back into its bowl and cover the entire bowl with cling film. Set aside to rest for about 30 minutes. After about 15 minutes, preheat your oven to 200C/fan 180C/gas.
Once the hour has elapsed, knead the dough again for about 30 seconds, then use your hands to form it into a log shape.
Use heavy-duty kitchen foil to tightly wrap the log. Twist the ends tightly and place it into a shallow baking dish in the middle of the oven.
Roast the Seitan for about 2 hours, turning after 1 hour, and carefully remove it from the foil when the roasting time had elapsed.
Add 2 tablespoon of vegan butter and 1 sliced red onion to a large frying pan over medium heat. Sprinkle in the black pepper and thyme sprigs and then place the Seitan log into the pan.
Pan-roast the Seitan for about 10-15 minutes constantly turning and use a spoon to keep pouring the butter mixture over the Seitan during this process. Set the pan aside to slightly cool.
Unroll the pastry sheet on a nonstick baking mat (or use kitchen foil!), place the log on one edge with the fried onions next to it, then roll the pastry sheet up. This has to happen quickly as the pastry becomes soft once the warm Seitan and onions touch it. Decorate your pastry to your liking and gently brush it with plant-based milk or melted vegan butter.
Place it in the oven for about 20 minutes or until the pastry has risen and turns golden brown.
Remove from the oven, slice, and serve with roast potatoes and your favourite trimmings!
Notes
If you're not used to cooking with puff pastry sheets, I recommend letting the Seitan cool down entirely before wrapping it in the pastry. That way you have more time in wrapping the log, and the pastry won't get soft that easily.
You can make the Seitan in advance and keep it in the fridge overnight or place it in the freezer.