Wash the rosemary sprigs and dab dry. Dab meat dry. Preheat oven (electric cooker: 80 °C/circulating air and gas: not suitable). Heat lard in a pan. Brown the fillet in it all around for about 5 minutes.
Season with salt and pepper. Spread rosemary on the fillet and tie up with kitchen string. Fry the fillet again briefly in hot lard all round. Put it on a baking tray. Cook in a hot oven for about 3 hours.
In the meantime, rinse the lentils in a sieve and drain them well. Heat the butter in a pot. Sauté the lentils for 1-2 minutes. Add stock and approx. 1⁄2 l water, bring to the boil, cover and simmer at low heat for 20-25 minutes until the lentils are soft.
Wash parsley, shake dry, pluck off leaves and chop finely. Stir the crème fraîche and mustard into the lentils. Season to taste with salt and pepper. Stir in parsley. Arrange the lentils with rosemary filet.
Boiled potatoes taste good with it.