Clean the mushrooms, wash them and cut them into thick slices. Peel and finely chop the onion. Roast the hazelnuts in a pan without fat, remove. Prepare noodles in boiling salted water according to package instructions, then drain.
Heat the oil in a frying pan. Fry the mushroom slices for about 4 minutes until golden brown. Shortly before the end of cooking, add the onion cubes and hazelnuts. Add milk and cream and bring to the boil. Stir in the sauce thickener and simmer for 3-4 minutes, stirring occasionally.
Season the sauce with salt, pepper and a little nutmeg. Clean the rocket, wash and drain well. Mix the noodles, 2/3 of the rocket and sauce and serve immediately. Sprinkle with remaining rocket.