01 - Preheat a grill pan or outdoor grill to medium-high heat.
02 - In a large bowl, toss zucchini, eggplant, bell peppers, and onion with olive oil, salt, and black pepper until evenly coated.
03 - Grill the vegetables for 3 to 4 minutes per side, turning occasionally, until tender and lightly charred.
04 - Spread a generous layer of pesto on the cut sides of each ciabatta roll. Layer grilled vegetables, mozzarella, and optional baby spinach on the bottom halves. Drizzle with balsamic glaze if desired.
05 - Top with the remaining ciabatta halves, place sandwiches on a preheated panini press or grill pan, and press for 3 to 5 minutes until bread is golden and cheese melts.
06 - Slice sandwiches in half and serve warm.