Preheat the oven to 190C/375F (fan).
Slice the croissants in half lengthways. Place a slice of cheese on the base of the croissants, followed by two slices of ham and another slice of cheese.
Place the croissants on a baking tray.
Mix the mustard, honey, and brown sugar together and brush on top of the croissants. Sprinkle with poppy seeds and place in the oven for 6-7 minutes to heat through.
Serve immediately.
