To prepare vinaigrette, combine first 5 ingredients; set aside.
To prepare the quesadillas, sprinkle steak with salt and 1/4 teaspoon pepper. Place the steak on grill rack coated with cooking spray, and cook 4 minutes on each side or until done. Let stand 5 minutes. Cut steak diagonally across grain into thin slices. Sprinkle 1/4 cup cheese evenly over each of 2 tortillas. Divide steak evenly over cheese; top with the remaining tortillas.
Heat a large nonstick skillet coated with cooking spray over medium heat. Cook quesadillas 4 minutes on each side or until golden brown. Remove the quesadillas from pan, and cut each into 8 wedges. Combine the vinaigrette, lettuce, onion, and tomato in a large bowl; toss well. Divide salad evenly among 4 plates; top each serving with 4 quesadilla wedges.