In a small bowl whisk together the oil, vinegar and mustard. Season with salt and pepper to taste.
Toss the spinach with the dressing until evenly coated then divide the spinach among 4 serving plates.
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. Serve immediately.
Per Serving: Calories 155; Total Fat 13 g; (Sat Fat 1.5 g, Mono Fat 6 g, Poly Fat 5.5 g) ; Protein 2 g; Carb 10 g; Fiber 3 g; Cholesterol 0 mg; Sodium 88 mg