The key to a good panino (singular) is high-quality bread, usually ciabatta, and fresh ingredients. Look for long, flat, rectangular-shaped loaves of ciabatta in your favorite deli or bakery.
1. Combine first 4 ingredients in a small bowl; stir well with a whisk.
2. Brush cut sides of bread with vinaigrette. Arrange prosciutto and next 3 ingredients over bottom half of bread; replace top half of bread. Cut loaf in half crosswise, and coat with cooking spray.
3. Heat a large grill pan or nonstick skillet over medium heat. Add sandwich halves to pan. Place a heavy skillet on top of sandwiches to weigh them down. Cook 3 minutes on each side or until bread is toasted and cheese melts. Cut each sandwich half in half again to form 4 equal portions. Serve immediately.
Note: Focaccia may be substituted for ciabatta.
Panini (pah-NEE-nee) translates as "rolls" or "little bread," but in Italy, the name is synonymous with sandwiches.