Wash the meat, dab dry, remove excess fat except for a small residue. Season meat with salt and pepper. Heat 2 tablespoons of oil in a flat roasting pan or an ovenproof pan.
Brown the meat on all sides for about 5 minutes. Cook in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for approx. 30 minutes. It is best to use a meat thermometer and cook up to 58 °C core temperature.
Boil eggs in boiling water for about 10 minutes until hard. Wash the potatoes and cook in plenty of boiling water for about 20 minutes. Quench eggs cold, peel and halve them. Remove egg yolks, pass through a sieve and stir with mustard until smooth.
Season with salt and pepper. Stir in 100 ml oil drop by drop. Gradually stir in sour cream and sour cream. Wash herbs, shake dry, pluck leaves from the stalks and chop finely, except for some parsley for garnishing.
Cut the chives into fine rolls. Cut the egg white into cubes and stir into the sauce, except for 1 tbsp. Remove the meat and let it rest. Drain potatoes, rinse with cold water and peel. Heat lard in a pan.
Fry the potatoes for about 10 minutes while turning. Season with salt and pepper. Cut meat into slices. Arrange meat, potatoes and sauce. Sprinkle with remaining egg white and herbs.