Clean and possibly wash mushrooms. Cut the mushrooms into slices. Halve oyster mushrooms if necessary. Clean and wash spring onions and cut them into pieces, except for something to sprinkle on the mushrooms. Heat oil in a pan and fry the mushrooms and spring onions.
Peel, wash and thinly slice the potatoes and carrots. Layer the potatoes, carrots, mushrooms and spring onions in a greased gratin dish. Season cream with salt, pepper and nutmeg and pour over the vegetables. Grate the cheese finely and sprinkle over it. Bake in the preheated oven (electric cooker: 200 °C/ gas: level 3) for 40-45 minutes. In the meantime defrost the onion mixture. Clean and wash the rocket. Whip oil under the vinegar and season with salt and pepper. Add onions and stir in. Mix rocket and vinaigrette.
In the meantime defrost the onion mixture. Clean and wash the rocket. Whip oil under the vinegar and season with salt and pepper. Add onions and stir in. Mix rocket and vinaigrette. Cut remaining spring onions into fine rings and sprinkle over the gratin. Serve together with the rocket salad