Clean, wash and cut the leek into rings. Melt the fat in a pot and sauté the leek in it. Dust with flour and sweat briefly. Gradually add 350 ml water and milk while stirring. Bring to the boil, stir in broth. Simmer for 6-8 minutes. Stir from time to time.
Stir 75 g cheese bit by bit into the sauce, season with salt and pepper. Fold the ham around 1 slice of bread. Layer ham sandwiches and sauce in a greased casserole dish. Sprinkle with 25 g cheese. Bake in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for approx. 15 minutes. Serve sprinkled with parsley