Preheat broiler with oven rack 4 inches from heat. Butterfly chicken breasts by making a lengthwise cut in 1 side of each, cutting through the opposite side; unfold. Sprinkle with salt and pepper.
Cook chicken in hot oil in a large sauté pan 5 minutes on each side or until well browned and done.
Transfer chicken to a baking sheet, and spread tops with mustard. Top with Swiss cheese and pancetta. Broil 3 minutes or until cheese is melted and pancetta is crisp. Serve immediately.