Wash the meat, dab dry and cut lengthwise into three. Whisk lemon juice and 1 tablespoon of oil and season with plenty of salt and pepper. Marinate the meat for about 30 minutes.
Peel and wash the potatoes and cut them roughly into pieces. Cover the potatoes and cook them for about 20 minutes in little boiling salted water. Warm up the milk. Drain potatoes. Add warm milk and mash.
Season mashed potatoes with salt and nutmeg.
In the meantime clean, wash and cut the leek into rings. Take the meat out of the marinade and let it drain. Heat 1 tablespoon of oil in a large frying pan. Fry the meat for approx. 8 minutes, turning it over.
Add the rest of the marinade and 100 ml of water to the pan, bring to the boil and dissolve the frying set. Add to meat and keep warm. Heat 1 tablespoon of oil in the pan and sauté the leek for about 4 minutes. Season with salt and pepper.
Mix about 2/3 leek into the puree. Arrange the puree and meat, sprinkle with the remaining leek, drizzle with sauce.