Preheat the oven to 350°F.
In a small bowl, mix the Dijon mustard, honey, and brown sugar until smooth and well combined.
Spread the honey mustard mixture evenly over the cut sides of all four croissants.
Place one slice of Swiss cheese on each croissant half. Add the deli ham to the bottom half of each croissant, using more or less according to your preference.
Place the croissant halves back together to form sandwiches.
Wrap each sandwich individually in aluminum foil and arrange them on a baking sheet.
Bake for 20–25 minutes, or until the sandwiches are heated through and the Swiss cheese is completely melted.
Carefully unwrap the hot croissants and serve immediately.