Spread a generous amount of pesto on two slices of thick bread.
Layer one half of the chicken breast on each slice of bread, then sprinkle slivered red onion, roasted red peppers, and mozzarella cheese evenly on top.
Place another slice of bread on each sandwich to close them up.
Preheat your panini press and lightly coat it with cooking spray.
Carefully place the sandwiches in the press and cook for about 3-4 minutes on each side.
Once the cheese is melted and the sandwiches are heated through, take them out and slice them in half. Serve warm and enjoy!