Preheat oven to 400°F Slice steak into thin strips.
Heat oil in a skillet over medium-high heat. Add steak, onions and 2 tablespoons fajita seasoning mix (Save remaining seasoning mix for another purpose). Cook onions and steak until desired doneness. Add 2 tablespoons water and bring to a boil. Remove from heat, set aside.
Place 8 tortillas on clean work surface. On one half of each tortilla, divide steak mixture, salsa (from kit), and avocado. Sprinkle with cheese. Fold tortillas in half. Place on baking sheet and bake until hot, about 5 minutes.