Lay out 6 square sheets of aluminum foil. Place the bottom half of a croissant on each sheet.

Spread a layer of Dijon mustard (and mayonnaise, if using) on the bottom half of each croissant.

Place one slice of Swiss cheese on top of the mustard layer.

Fold the thinly sliced deli ham and pile it evenly onto the 6 sandwiches.

Top the ham with a second slice of Swiss cheese, then place the top half of the croissant on the sandwich.

Gently fold the aluminum foil up and around each croissant, completely wrapping it without crushing the pastry.

To bake immediately: Preheat oven to 350°F (175°C). Place wrapped sandwiches on a baking sheet and bake for 15-20 minutes until hot and cheese is melted.

To make ahead: Store the foil-wrapped sandwiches in the refrigerator for up to 3 days, or freeze in a large ziplock bag for up to 2 months. Bake chilled sandwiches for 20 minutes, or frozen sandwiches for 25-35 minutes at 350°F.

Carefully unwrap the hot sandwiches and serve immediately.