Roast Beef and Caramelized Onion Panini

With long, slow cooking, onions become meltingly sweet and caramelized, lending luscious flavor to our roast beef panini. This recipe for caramelized onions yields more than you will need for one panini. Use the rest to make additional sandwiches, stir into mashed potatoes or pasta, or use as a topping for pizza. The onions can be stored in an airtight container in the refrigerator for up to 1 week.