IF MAKING STEAK: Liberally sprinkle all sides of the steak with taco seasoning. Preheat grill or grill pan to medium high. Cook for 5 on each side. Remove from heat and let rest at least 10 minutes before cutting into thin slices. IF USING LEFTOVER STEAK: Thinly slice steak and sprinkle with ½ tbsp of taco seasoning.
Mix together remaining taco seasoning and softened cream cheese in a bowl.
Lay tortilla flat and smear on half of the tortilla with the seasoned cream cheese. Sprinkle cream cheese side with steak pieces, cheddar cheese, spinach ribbons, and then mozzarella cheese. Fold blank tortilla side over the loaded side and press down slightly.
Preheat griddle or flat pan to high.
Spray the cooking surface with a good amount of cooking spray and add built quesadillas. DO NOT CROWD - if you need to cook in batches, please do.
If cooking on griddle, cook for 4 minutes on each side. If using flat pan, for 5 minutes on each side. Just before flipping each quesadilla, spray the top with cooking spray. Cut in thirds to serve.