For the Frankfurt Loin Roast, score the beef tenderloin lengthwise so that it can be rolled up like a roulade.
Peel the mushrooms, peel the onion and chop finely together with the parsley. Sauté in the butter. Leave to cool.
Spread the roasted vegetables evenly on the meat platter and place the chicken or turkey liver pieces as a strand in the middle, season with salt and roll up. Tie with twine. Season the roll with pepper, brush with mustard and season with salt. Fry in hot oil on all sides.
Then roast in the oven for 30 minutes at 200 degrees. While doing this, baste a few times with its own juices and gradually pour the clear soup.
Remove the meat from the roasting pan, wrap in foil and keep warm.
Boil the meat stock, add some clear soup if necessary and put it into a small pot. Add the whipped cream, season with a little lemon juice and simmer for a few minutes.
Arrange the Frankfurter Lung Roast and serve.