In a hot skillet, saute the seitan in hot olive oil. When brown (about 2 to 3 minutes), gently flip it over with tongs. Add garlic to the side of the seitan and let it brown with the seitan in the oil. Add salt and pepper to taste.
When the garlic has started to brown, cover the whole pan with the mushrooms. Remove from heat and add the Marsala. Return to heat, add the stock and fresh herbs.
When the mushrooms have cooked down, slowly stir in the slurry. Boil for about 30 seconds. Stir and serve.