1. Combine first 7 ingredients in a medium bowl, stirring well to coat. Place 4 bread slices on a flat surface; top each bread slice with 1 cheese slice. Divide tuna mixture evenly among bread slices; top each serving with 1 remaining bread slice.
2. Heat a large skillet over medium heat. Lightly coat sandwiches with cooking spray. Place sandwiches in pan; top with another heavy skillet. Cook 3 minutes on each side or until lightly browned (leave skillet on sandwiches as they cook).