For the mushroom sauce, clean the mushrooms, wash them briefly if necessary and cut them into slices. Wash the sage, dab dry and cut the leaves into strips. Peel the onions. Dice one finely, halve the other and cut into thin rings.
Heat 1 tablespoon of oil in a frying pan. Fry the mushrooms for about 5 minutes, turning them until golden brown. Fry onion cubes and sage briefly. Season with salt and pepper. Dust with flour and sauté until light brown.
Deglaze with 350 ml water and cream. Bring to the boil, stir in the vegetable stock and simmer for 3-4 minutes while stirring. Season to taste with salt and pepper. Keep sauce warm.
For the Rösti, grate the parmesan in a large bowl. Peel the potatoes, wash and coarsely grate them. Add the onion rings. Add egg and starch, knead everything. Season with salt, pepper and nutmeg.
Fry the ham in a pan without fat until crispy. Remove. Heat 1 tablespoon of oil in the pan. Fry 3 rösti on each side for 2-3 minutes until crispy. Remove and keep warm in a hot oven (approx. 50°C).
Fry another 5-6 Rösti in 2 portions in 1 tablespoon of oil each. Arrange everything.