Chicken and Gruyère Sandwiches with Bacon and Balsamic Onions

This sandwich is delicious served at room temperature, but we think it becomes something special when you grill it in a panini press. If you don’t have a panini press, grill the sandwiches in a cast-iron pan, preferably with a weight on top. For a party, slice the rolls crosswise into four or five pieces so guests can pile them on their plates with other small bites.