Toast the walnuts in a skillet over medium heat for 3-4 minutes, or until brown and fragrant.
While the walnuts are toasting, slice the apple into matchstick pieces.
Place greens and apples in a large bowl.
In a small bowl, whisk together the vinegar, honey and mustard and then slowly whisk in the oil. Drizzle over the salad and toss gently to coat.
Divide the salad among the plates, sprinkle each with walnuts and serve.