Place deep-frozen Rösti next to each other on a baking tray. Cook in the preheated oven (electric cooker: 200 °C/ gas: level 3) for about 20 minutes. In the meantime, wash, clean and halve the mushrooms. Peel and finely chop the onions. Heat the fat. Fry the mushrooms and onions over a high heat for about 10 minutes, turning them over. Season with salt and pepper.
Dust with flour. Add cream and stock while stirring constantly, let simmer for 2-3 minutes. Season to taste again. Wash and chop the parsley and sprinkle over it. Put 1 Rösti in each raclette pan. Spread creamed mushrooms in portions on top. (Add other ingredients as you like.) Cover with cheese slices and bake