01 - Preheat a grill pan or outdoor grill over medium-high heat.
02 - Brush zucchini, eggplant, and bell peppers with olive oil and season with salt and black pepper.
03 - Grill vegetables for 2 to 3 minutes per side until tender and lightly charred, then transfer to a plate.
04 - Slice ciabatta rolls in half horizontally and spread 1 tablespoon of pesto on the cut side of each bottom half.
05 - Layer grilled vegetables evenly over pesto, then top with mozzarella slices and sprinkle with Parmesan if using.
06 - Place top halves of the rolls on the sandwiches.
07 - Heat a large skillet or panini press over medium heat. If using skillet, place sandwiches in pan, press with a heavy skillet or spatula and cook for 2 to 3 minutes per side until bread is golden and cheese has melted. If using panini press, grill according to manufacturer's instructions.
08 - Slice panini in half and serve warm.