1Heat panini maker or closed contact grill for 5 minutes. Spread 1 side of each bread slice with butter. On unbuttered side of 2 slices, arrange half of the cheese. Top evenly with chicken, bacon, tomato, onion and remaining cheese. Top with remaining 2 bread slices, buttered sides up.
2Place sandwiches on grill. Close grill; cook 3 to 5 minutes or until bread is toasted and cheese is melted. Cut each sandwich in half to serve.