Yes, you can reheat a Croque Monsieur. If you have leftovers, place in an airtight container. To reheat it, preheat your oven or your air fryer to around 350°F (175°C). Place the Croque Monsieur on a baking sheet and cover it loosely with aluminum foil to prevent it from drying out. Reheat it in the oven for about 10-15 minutes (5 minutes in the air fryer), or until it is heated through and the cheese is melted and bubbly. You can also use a sandwich press.