Lay a slice of bread on your work surface. Using a palette knife, spread a layer of béchamel over the slice, then place 2 slices of ham on top. Add a second layer of béchamel then place another slice of bread on top, pressing lightly on it. Place another slice of ham on top, then spread a generous layer of béchamel over the ham.
Repeat with the remaining ingredients to make 6 croque-monsieurs in total and then place them on a baking tray. Sprinkle the grated Aletsch or Gruyère cheese over the top of them and press it lightly into the béchamel so that it sticks well. Cover the tray with cling film and refrigerate until ready to serve.
Preheat the oven to 180°C/Gas Mark 4. Bake the croque-monsieurs in the oven for 15 minutes. Serve on individual plates, either whole, as they are, or cut into triangles.