Preheat your oven to 350°F (175°C).
Slice the croissants in half horizontally.
In a small bowl, mix together the Dijon mustard and mayonnaise. Spread this mixture evenly on the cut sides of each croissant.
Layer the sliced ham on the bottom half of each croissant, followed by the sliced cheddar cheese.
Add a handful of arugula or spinach and a few slices of tomato on top of the cheese. Season with salt and pepper to taste.
Close the sandwiches with the top halves of the croissants.
Place the assembled sandwiches on a baking sheet and bake in the preheated oven for about 10-12 minutes, or until the cheese is melted and the croissants are golden brown.
Remove from the oven and let cool for a minute before slicing in half and serving.