Place a large nonstick skillet coated with cooking spray over medium heat. Carefully arrange 4 onion slices in skillet, and sprinkle with 1/2 teaspoon sugar; sauté 4 minutes on each side or until golden brown. Wipe skillet; repeat procedure with remaining onion slices and 1/2 teaspoon sugar.
Combine salad greens, apple, and cheese. Combine water, vinegar, oil, salt, and pepper; pour vinegar mixture over salad green mixture, tossing to coat. Divide salad evenly among 4 plates; top each serving with onion slices.