Slice your croissants in half and scrape with mustard. Top with a slice of ham then some emmental cheese. Squash them with your hands. Lay 5 in the bottom of a skillet buttered with your butter. Top with a little bechamel, a bit of torn ham and a fistful of emmental. Cover with another layer of croissants then top with the remaining sauce and cheese. Grate over some nutmeg and bake at 180°C for 25 minutes, or until coloured, bubbly and melted.