Freeze 25 g butter in small pieces. Peel, wash and cut the potatoes into pieces. Cook in boiling salted water for about 20 minutes. Dab medallions dry and season with salt. Heat oil in a pan, fry medallions on each side for about 5 minutes.
Clean the leek, halve and wash lengthwise, cut into fine strips. Remove medallions from the pan, season with pepper and keep warm. Fry leek in frying fat for 2-3 minutes. Season with salt and pepper.
Melt 150 g butter in a frying pan and roast the pancob crumbs in it until golden brown.
Bring syrup, soy sauce, 100 ml water, miso paste and mustard to the boil in a small pot. Gradually add ice-cold butter to the simmering sauce. Drain the potatoes and let them evaporate briefly. Add milk and 2 tbsp. butter, season with salt and nutmeg and mash to a puree.
Arrange the puree, sprinkle with leek and crumbs. Serve with medallions and sauce.