How to Make It
To prepare vegetables, preheat grill to medium-high (about 450°F). Coat first 6 ingredients with cooking spray; arrange on grill grate. Grill green onions 2 minutes; remove from grill and chop. Grill remaining vegetables 5 more minutes, turning occasionally. Remove stems and seeds from bell peppers; discard. Reserve 4 bell peppers and 4 red onion wedges; cut remaining vegetables into 2-inch pieces.
To prepare panini, place reserved 4 bell peppers and 4 red onion wedges in food processor with olives, capers, 1/2 teaspoon black pepper, and ground red pepper; process until smooth. Hollow out top and bottom halves of rolls, leaving a 1/2-inch-thick shell. Spread bell pepper mixture over bottom halves of rolls; top with remaining grilled vegetables, mozzarella, basil, and top halves of rolls.
Coat grill grate with cooking spray. Place 2 sandwiches on grill; top with a cast-iron skillet. Grill 2 minutes on each side or until cheese melts. Repeat procedure with remaining 2 sandwiches.