Peel, wash and cut the potatoes into small pieces. Cook in plenty of boiling salted water for about 15 minutes. After about 7 minutes, add the croissant noodles and quickly bring everything back to the boil.
Preheat oven to 175 degrees.
Grate the cheese. Bring the cream and milk to the boil, melt 2/3 of the cheese in it while stirring, season the sauce with salt and pepper. Drain the potatoes and pasta.
Grease a large baking dish. Pour in the potatoes and pasta and pour the cheese sauce over it, sprinkle with the remaining cheese. Bake for about 5 minutes until the cheese has melted.
Peel the onions and cut into rings. Heat the oil in a pan and fry the onions until golden brown.
Take the casserole out of the oven, place the onion rings on top and serve the casserole sprinkled with parsley.