Place the Rösti on a baking tray lined with baking paper and bake in a preheated oven (electric range: 200 °C/ convection oven: 175 °C/ gas: level 3) for 15 minutes. Clean the oyster mushrooms with kitchen paper, halve if necessary.
Peel and finely dice the onions and garlic. Heat the oil in a pan. Sauté the oyster mushrooms in it. Add onions and garlic. Season with salt, pepper and herbs from Provence. Pour on white wine, 200 ml water and cream and bring to the boil.
Wash, halve and add the tomatoes. Stir in sauce thickener, bring to the boil again. Season to taste with salt and pepper. Garnish with marjoram and rosemary.