Place onion in a zip-lock bag. Add enough vinegar to completely coat the onions. Marinate at least 2 hours but preferably 6+ hours. Drain vinegar.
In large bowl, combine greens, tomatoes, croutons, prosciutto, peppers, nuts and the onions.
Mix together the dressing and the garlic. Pour over salad; toss to coat.
Divide onto serving plates. Top with plenty of fresh grated or shaved Parmesan cheese.
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