Pat the pre-cut fajita strips dry with paper towels to ensure a good sear and set aside.
Slice bell peppers, onions, and mushrooms into thin strips. Remove the seeds from the bell peppers.
In a small bowl, mix chili powder, smoked paprika, Mexican oregano, minced onion and garlic, ground cumin, salt, pepper, and a pinch of cayenne pepper. Set aside.
Heat a large cast iron skillet over medium-high heat with two tablespoons of oil or butter. Add the vegetables and cook for 5-7 minutes, stirring occasionally. Add half of the seasoning mix during the last two minutes of cooking. Remove the vegetables from the skillet.
Return the skillet to medium heat and add two more tablespoons of butter. Add the pre-cut fajita strips, sprinkle with the remaining seasoning, and cook for 3-4 minutes, stirring frequently until cooked.
Return the vegetables to the skillet once the steak is done. Toss everything together and serve hot with tortillas and your favorite toppings.