Heat oil on pan over medium-high heat. Once the pan is hot, add the peppers, onions, and mushrooms to the pan (it should sizzle... A LOT).
Add in the liquid smoke and taco seasoning. Stir consistently (you'll want to remain at the stove until cooked through).
Once veggies are visibly charred and cooked through to your liking, remove them from the eye or turn to low heat.
In another pan, heat your tortillas over medium heat.
Top your tortillas with a generous serving of the fajita mix and enjoy!