Soak long bamboo skewers in water for at least 5 minutes. Thread skewers with shrimp (2), grapes (6) and onions (2), with a ribbon of prosciutto winding throughout. Place the finished skewers in shallow dish or plate. Evenly dab on half of the marinade with a brush. Let marinate in the refrigerator for at least 30 minutes and then brush with the remaining marinade. Grill on a hot fire for 2 minutes a side. Serve immediately.

To make the marinade: whisk together the garlic, vinegar, rosemary, salt and pepper and gradually add the oil.