Add all of the vegan seitan steak ingredients except the vital wheat gluten to a food processor and pulse to combine, stopping to scrape down the sides as needed until everything is well mixed.
Pour the mixture into a bowl with the vital wheat gluten and knead together. It may be crumbly at first, but keep kneading it until it comes together into a tight ball. Cut the ball into four sections. Use your fingers to press each section about ½-inch thick.
In a pot with a steamer basket, bring one cup of water to a boil. Wrap the seitan dough loosely with aluminum foil and place in the steamer basket. Cover the pot and steam for 35 minutes.
Meanwhile, in a large bowl, combine all the marinade ingredients. Remove the cooked steaks from the steamer, unwrap and coat them in the marinade for at least one hour and up to overnight.
Heat a grill pan over medium-high heat. When hot, sear the steaks for a couple of minutes per side. Brush the steaks with leftover marinade while cooking to keep them juicy.
Serve the seitan steaks with roasted potatoes (optional), parsley, and tomatoes along with the remaining marinade.