Place mushroom slices and onion in a large non-stick skillet at medium high heat. Heat for about 5 minutes until liquid begins to release for mushrooms and onions begin to become translucent.
Add water and fajita seasoning packet. Stir to combine.
Add bell pepper strips to skillet and heat for about 10 minutes until veggies are soft and flavor has distributed evenly.
Remove from heat.
Assemble fajitas by adding veggies to tortillas and topping with whatever garnishes you desire. Fresh avocado, salsa, sour cream, or cheese are great options.