Slice the mushrooms, peppers, and onion and place into a large container with a lid. I like my mushrooms and peppers slice fairly thick, and the onions sliced thinly, but you can go by your own preferences.
Toss the remaining ingredients (3 tbsp oil, 5 tbsp soy sauce, 1 tbsp vegan Worcestershire sauce, 1 tbsp lime juice, 1 tbsp taco seasoning, and 1 ½ tbsp. nutritional yeast) onto the sliced vegetables.
Place a lid on the container and shake the vegetables well so the marinade covers all slices. Place into the fridge to marinate overnight if possible (aim for at least 2 hours).
When the vegetables have marinated and you are ready to cook, preheat the oven to 400 °F.
Place an even layer of vegetables on 2-3 baking trays. Bake for 30 minutes, tossing halfway through, until the vegetable edges are cooked and crisping. Some people prefer more “blackened” fajitas; you can achieve this by leaving the vegetables in the oven for longer (an additional 15-20 minutes) or turn your broiler on and place each tray (one at a time) under the broiler for 2-3 minutes. Do not leave your oven unattended with the broiler on – food can burn within a minute or two so keep a very close eye on things if you choose to do this.
While the fajitas cook, prepare the toppings and other ingredients to serve.
Accuracy of nutrition information cannot be guaranteed (certain ingredients may not have all nutrients listed in the database); amounts may vary (and will vary depending on brands of ingredients used); all nutrition fact values rounded to the nearest whole number.Nutrition information only based on fajita vegetables included in the recipe; does not include tortillas, avocado, sour cream or other toppings.Nutrition information assumes the use of low-sodium soy sauce or tamari.