1) In a small bowl whisk together the oil, vinegar and mustard. Season with salt and pepper to taste.
2) Toss the spinach with the dressing until evenly coated then divide the spinach among four serving plates.
3) Core the apple and slice it into matchsticks. Sprinkle a quarter of the apple pieces on top of each salad. Follow with the walnut pieces.
4) Serve immediately.