How to Make It
Combine mayonnaise, blue cheese, horseradish, and 1/8 teaspoon black pepper in a small bowl; cover and refrigerate 30 minutes.
Heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray. Sprinkle steak with salt and remaining 1/8 teaspoon pepper. Add steak to pan; sauté 4 minutes or until desired degree of doneness. Let stand 5 minutes; cut into 1-inch pieces.
Combine the lettuce, tomatoes, and onion in a bowl. Spread about 2 tablespoons mayonnaise mixture evenly over each tortilla. Divide steak and lettuce mixture evenly among tortillas; roll up.