Start by pouring 160ml boiled water over the dried mushrooms and leave to stand for at least 20 minutes. Stir the mushrooms around a few times throughout this time to really release the flavour into the liquid. Once this is ready remove the actual mushrooms and keep just the liquid for use in the wet mix of the seitan (save the mushrooms to use in the sauce or for another recipe!)
For the seitan add all of the dry ingredients to a mixing bowl and stir well to combine. Add all of the wet ingredients to a blender and blend until smooth. (You do not need to add the dried mushrooms, just the liquid they rehydrated in).
Pour the wet mix into the dry and stir until combined into a dough. The dough then needs to be kneaded vigorously for 10-12 minutes to activate the gluten which forms the fibres giving a meaty texture. Every couple of minutes or so I stop kneading and flatten the dough out either by bashing with a rolling pin or just punching it. This really helps develop the right texture. Once you're happy the dough is kneaded enough make sure to flatten it out again ensuring its is only 1-2 inches thick.
You can then cut the dough into steaks. I managed to get 5 steaks but it depends how big you want each one. (Optional step: Leave the steaks to firm up for an hour. This will result in a firmer texture, but as long as you have kneaded for long enough you will already have firm steaks).
When you are ready to start cooking your seitan bring 2L of veg stock to a simmer in a large pot. Quarter the onion and add it to the stock along with the rosemary, thyme, marmite, red wine, salt and pepper and stir to everything in. Add in your steaks bearing in mind they will expand in the liquid, so try not to overcrowd the pot, I fit two at a time. Simmer for 40-45 mins. Remove from the pot after this time, the steaks can be used immediately, but they will gain a firmer texture if you leave them to cool for an hour.
Either way, once you are ready to cook them start by making the peppercorn sauce. Heat a sauce pan over a medium-high heat and a frying pan on a high heat and add the mushrooms and cook these for 5-10 minutes before adding the garlic and mustard. Stir everything so it is coated in the mustard and then add the red wine and season with salt and add the peppercorns.
Once the alcohol has cooked away, you can then add the veg stock and season with black pepper. Allow the sauce to thicken a little, then use a brush to glaze over the steaks on both sides. Return the sauce pan to the heat and add the cream and stir everything to combine and allow to cook for a minute more. During this time you can cook the steaks for 1-2 minutes on each side in the frying pan.