Peel and chop the onion. Heat the oil in a large pan. Fry the bacon for 2-3 minutes. Add onions and fry briefly. Add 300 g cream, season with pepper and a little salt.
Add the noodles and let them stand in the hot cream for 1-2 minutes. Remove from heat. Whisk egg with 100 g cream. Stir the parmesan into the sauce and mix well. Season to taste once more. Put the pasta with the sauce in an ovenproof dish.
Bake in the preheated oven (electric oven: 200 °C/circulating air: 175 °C/gas: level 3) for 15-20 minutes. Wash the parsley, dab dry. Cut into fine strips. Sprinkle over the gratin and serve.