Steps
1. Preheat a grill or grill pan over medium heat.
2. Brush the zucchini, red bell pepper, and red onion slices with olive oil, and season with salt and pepper.
3. Grill the vegetables for 3-4 minutes on each side until tender and slightly charred.
4. Remove the vegetables from the grill and set aside.
5. Preheat a panini press or grill pan over medium heat.
6. Assemble the sandwich by placing the grilled vegetables and mozzarella cheese between the slices of bread.
7. Brush the outside of the sandwich with olive oil.
8. Grill the sandwich in the panini press or grill pan for 3-4 minutes on each side, until the bread is golden and crispy, and the cheese is melted.
9. Remove the sandwich from the press or pan and let it cool for a minute.
10. Slice the panini in half and serve hot.