Directions

1. For stuffing, in a large skillet melt butter. Add mushrooms, onion, and thyme; cook until onion is tender. Remove from heat; stir in crushed croutons, prosciutto, cheese, and the 1 tablespoon balsamic vinegar. Season with pepper. Meanwhile, in a small saucepan, bring remaining balsamic vinegar to boiling. Boil, uncovered, until reduced by half, about 6 minutes; remove from heat.
2. Prepare medium-hot banked fire in covered grill or preheat gas to medium-high. Lightly oil grill grate.
3. To butterfly the pork loin, place fat-side down. Make a single lengthwise cut down the center of the pork, cutting to within 1/2 inch of the bottom side. Spread open. Make another lengthwise slit to the right of the first cut and a third slit to the left. Cover pork with clear plastic wrap. Pound with a meat mallet to even thickness, about 3/4 inch. Remove plastic wrap.