Place the top sirloin steak in the freezer for 30-45 minutes. Remove from the freezer and slice the steak into very thin pieces. The thinner, the better! If the steak is too frozen, allow it to thaw slightly before slicing.
In a large bowl, toss the thinly sliced steak with 1 tbsp olive oil, 2 tsp salt, and 1 tsp ground black pepper.
In another bowl, toss the sliced bell peppers and onions with the remaining 1 tbsp olive oil, 1 tsp salt, and 1 tsp ground black pepper.
Preheat the air fryer to 400˚F / 205˚C. Place the entire pepper and onion mixture into the air fryer, spreading it out in one layer. Air Fry for 10-13 minutes, or until the onions and peppers have wilted and some pieces are lightly charred. Set aside.
Preheat the air fryer to 400˚F / 205˚C again. Add the steak strips in batches, ensuring not to crowd the basket. Air Fry for 8-10 minutes, or until the steak is cooked through and browned along the edges.
Meanwhile, prep the quesadillas. On a clean work surface, lay out four tortillas and sprinkle ½ cup of the cheese on top of each tortilla. Place another tortilla on top of each cheesy tortilla. Finish by sprinkling an additional ¼ cup of cheese on top of each quesadilla.
Preheat the air fryer to 350˚F / 175˚C. Carefully place two quesadillas inside the air fryer. Air Fry for 6-8 minutes, or until the cheese has melted on top and inside of each quesadilla. Repeat with the remaining quesadillas.
To assemble, spoon ¼ of the onion and pepper mixture onto each cheesy-hot quesadilla. Top each with ¼ of the steak mixture. Sprinkle with sliced scallions and drizzle with lemon juice. Serve warm for delicious cheese pulls!