Wash the rosemary and shake dry. Dab meat dry, wrap with kitchen string. Heat oil in a pan. Brown the meat in it for about 5 minutes.
Season with salt and pepper. Put rosemary between meat and kitchen string. Fry briefly more. Cook in the preheated oven (electric cooker: 100 °C/ convection oven: not suitable/ gas: not suitable) for approx. 50 minutes
Place the lentils on a sieve, rinse and drain. Heat butter in a pot. Sauté the lentils for 1-2 minutes. Add the stock and about 500 ml water, bring to the boil and cook over a low heat for 20-25 minutes until the lentils are soft. Stir in crème fraîche and mustard, season with salt and pepper.
Wash the parsley, shake dry. Peel and chop the leaves. Stir the parsley into the lentils, except for 2 tablespoons. Arrange lentils and meat, sprinkle with remaining parsley