Preheat the oven to 200 ° C fan oven. Prepare pasta firm to the bite according to the instructions on the packet. Put the frozen forest mushrooms in a small saucepan and bring to the boil without adding water, then reduce the heat, stir in the sour cream, pepper, salt and vegetable stock, finely chop the parsley and mix in. Let simmer for 10 minutes. Now grease a casserole dish of a suitable size with the butter, put the pasta and mushrooms with the stock in the dish, sprinkle with the Gruyere and put in the oven at 200 ° C for 20 minutes.
The gratin tastes very good with roasted game. Strauss also goes well.