Prepare the vinaigrette by combining the balsamic vinegar, lemon juice, maple syrup, 2 teaspoons of sea salt, and the thyme in a medium bowl.
Add the olive oil and set aside.
Season the steak with the remaining salt and pepper.
Spray a medium skillet with cooking spray.
Add the steak and brown for 3 to 4 minutes on each side.
Transfer the steak to a cutting board.
Combine the lettuce and watercress in a large bowl.
Add the tomatoes and onion.
Toss the salad with vinaigrette dressing.
Cut the steaks crosswise into thin slices.
Arrange the steak slices on top of the salad and drizzle more vinaigrette over the steak.
Serve.