Clean mushrooms, rub with kitchen paper. Halve or slice the mushrooms according to size. Cut oyster mushrooms a little smaller. Peel, dice or crush shallots and garlic.
Clean, wash and cut the leek into rings. Steam about 5 minutes in little boiling salted water. Cut ham into strips. Heat clarified butter in a large coated pan. Brown shallots and garlic in it until golden brown.
Add the mushrooms and fry vigorously, turning several times. Drain leek on a sieve (collect approx. 1/2 cup of vegetable water) and mix into the mushrooms. Add ham strips and vegetable water and season to taste with salt, pepper and marjoram.
Put the cheese on the mushroom vegetables and let it melt with the lid of the pan closed. Serve garnished with parsley. Serve with a hearty farmhouse bread.