Place spinach in large bowl, set aside. In small bowl, combine balsamic vinegar, olive oil, salt and pepper. Set aside. In large saute pan, cook pancetta over medium-high heat until almost crisp, about 5-7 minutes. Do not drain fat. Add pine nuts and cook, stirring constantly for about 1 minute. Add grapes and red onions; continue to cook for 1 minute. Add balsamic dressing and bring to a boil, about 30 seconds. Turn off heat and pour hot dressing over spinach. Toss quickly to coat leaves. Salad should be barely warm. Sprinkle with parmesan shavings and serve.
Nutritional analysis per serving: Calories 294; Protein 8 g; Fat 26 g; Calories from Fat 76%; Carbohydrates 10 g; Cholesterol 19 mg; Fiber 2 g; Sodium 430 mg.