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.