To make balsamic vinaigrette: Add olive oil, balsamic vinegar, and honey Dijon mustard to a bowl. Whisk to combine ingredients.
Toss balsamic vinaigrette with mixed salad greens to coat. Arrange the salad greens in a bowl.
Layer cucumber, red onion, fresh herbs, watermelon, and feta cheese over the mixed salad greens. Serve salad immediately.