Slice the seitan into thin 1/2-inch cutlets and season both sides evenly with sea salt and black pepper.
Heat the extra virgin olive oil in a large stainless steel skillet over medium-high heat.
Place the seitan cutlets in the pan and sear for 3-4 minutes per side until they develop a deep golden-brown crust.
Remove the seitan from the skillet and set aside on a plate.
In the same pan, add the sliced wild mushrooms and minced shallots, sautéing for 5 minutes until the mushrooms release their moisture and begin to brown.
Add the minced garlic and cook for another 60 seconds until fragrant.
Pour in the dry Marsala wine to deglaze the pan, using a wooden spoon to scrape up all the flavorful browned bits from the bottom.
In a small bowl, whisk the arrowroot powder into the cold vegetable broth until smooth, then pour the mixture into the skillet.
Simmer the sauce for 2-3 minutes until it thickens into a glossy, velvety consistency.
Return the seitan cutlets to the pan, spooning the mushroom sauce over them to heat through.
Garnish with freshly chopped parsley and serve immediately.