Heat closed contact grill 3 to 5 minutes.
Cut each piece focaccia in half horizontally. Spread 1 tablespoon pesto on cut sides of each half. Layer bottom halves with one-fourth of the chicken, bell pepper, onion and cheese. Cover with remaining focaccia halves.
When grill is heated, place sandwiches on grill. Close grill; grill 3to 5 minutes or until focaccia is golden brown and cheese is melted.