Coat both sides of bread with cooking spray; toast. Cut into 1-inch pieces.
Sprinkle tuna with salt and pepper; coat with cooking spray. Heat a nonstick skillet coated with cooking spray over medium-high heat; cook tuna 2 minutes on each side or until desired degree of doneness. Cool 5 minutes; cut into 1-inch cubes.
Combine dressing and tarragon in a small bowl. Combine greens, tomatoes, and onion in a bowl; toss with dressing. Add tuna and bread; toss well.