You can make your own pesto for these sandwiches using our recipe, or if you prefer, look for ready-made pesto in the refrigerated section at the grocery store, near the fresh pasta sauces.
Toss mozzarella and pesto in a medium bowl until cheese is completely coated. Split rolls in half horizontally. Cover bottom half of each roll with cheese cubes. Top with lettuce and green pepper; return tops of rolls. Cut sandwiches into smaller pieces, if desired, and serve.