Preheat oven to 400°F and spray nonstick cooking spray on sheet pan (I use avocado oil cooking spray).
Slice the onion and bell peppers into thin strips and arrange on sheet pan, leaving room for portabella mushroom slices.
Arrange the portabella mushrooms on the sheet pan.
Drizzle the portabella mushrooms, bell peppers and onion with avocado oil and season the entire sheet pan with the 2 Fajita Time! seasoning packets included with the mushrooms. Mix to coat all of the veggies.
Cook for 20 minutes, turning veggies at the 10 minute mark so they cook evenly.
Remove sheet pan from oven after 20 minutes and serve on tortillas with your toppings of choice. Enjoy!