Generously spread a thick layer of pesto over two slices of Italian bread, ensuring an even coating for maximum flavor. Lay one half of the cooked chicken breast atop each pesto-covered slice, followed by a scattering of slivered red onion and roasted red peppers. Evenly distribute the mozzarella cheese over the assembled ingredients before placing the remaining bread slices on top to form a sandwich.

Preheat a panini press and lightly coat the grilling plates with cooking spray to prevent sticking. Once heated, carefully place the sandwiches inside and close the press. Grill for 3 to 4 minutes per side, adjusting the time based on the thickness of the bread and the intensity of the press.

When the exterior is crisp and golden, and the cheese is thoroughly melted, remove the sandwiches from the press. Using a sharp knife, slice each panini in half and serve immediately while hot.