Cut the croissants through the middle so you have a top and a bottom. Pre-heat the broiler/grill. If you'd like to crisp up the croissants a bit, place them outside first under the broiler/grill for a couple minutes, then turn and toast the inside, keeping a close eye on them as they can easily burn.

Spread a thin layer of mustard on the inside of the bottom of the croissants and lay the ham on top - you may need to cut the ham to get it to fit a little better. Cover the ham with grated cheese (this is the approx ½cup/45g) so you have a good layer but it's not falling off.

Make the sauce by melting the butter in a small pan. When melted, add the flour and cook, stirring regularly, for a couple minutes until it tarts to smell slightly nutty.

Gradually add the milk to the butter-flour base, a little at a time and stirring as you go so it is well combined and trying to avoid lumps. Allow the sauce to warm through and thicken up. Once it is relatively thick (it will still run off your spoon but more slowly), add the ⅓cup/30g cheese and stir in so it melts. Add a little nutmeg and pepper to taste, remove from heat and set aside.

Toast the bottom half of the croissant (with the ham and cheese on) so the cheese melts. Meanwhile spread a spoonful of the sauce on the inside of the tops of the croissants. When the cheese has melted, put the top of the croissants on top of the bottom then pour the sauce over the top of the croissants. If you haven't had your croissants sitting on a tray you might want to before you do this so you don't lose any sauce. Toast the croissants under the broiler/grill until the sauce starts to brown slightly.

Remove and serve, scooping any sauce that ran off over the croissants, optionally topped with a few chives.