For the sauce, cut 60 g butter into small pieces and freeze. Peel and wash the potatoes and cut them into pieces. Cook in boiling salted water for about 20 minutes. Clean, wash and cut leek into fine rings.
Heat 2 tablespoons of butter in a large frying pan. Sauté the leek for 2-3 minutes. Season with salt and pepper and remove. Heat the oil in the pan. Dab medallions dry and fry on each side for 3-4 minutes.
Season with salt and pepper. Remove, keep warm. Melt 100 g butter in a frying pan and roast pancob crumbs in it until golden brown.
Bring syrup, soy sauce and 175 ml water to the boil in a small pot. Drain the potatoes, add the milk and mash them with a potato masher until mashed. Season to taste with salt and nutmeg. Fold in the leek loosely.
Remove 4 tbsp. from the boiling sauce and mix with mustard and miso paste. Remove the rest of the sauce from the stove and stir in first the miso mustard mix and then the frozen butter. Serve the sauce immediately with medallions and puree, pour pancob crumbs over the puree.