Clean and wash the cucumber and cut into very thin slices. Mix sour cream, 125 g cream, vinegar, salt, pepper, sugar and dill. Mix the cucumber and the Schmanddressing and let it stand in a cool place.
Wash the fillets, dab dry with kitchen paper and cut each fillet into about 6 medallions. Mix mustard, breadcrumbs, parsley and pepper. Heat the oil in a large pan and fry the medallions on each side for about 2 minutes.
Remove from the pan, place on a baking tray lined with baking paper and spread the mustard-herb mixture on top. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 10-12 minutes.
Peel and finely dice the onion and add it to the cooking fat. Fry until translucent, turning the onion. Deglaze with stock and 100 g cream, bring to the boil, stir in sauce thickener. N
In the meantime wash parsley, dab dry and chop finely. Pour 4 cups of boiling water into a bowl. Stir in the puree flakes well with a wooden spoon. Stir again briefly before serving.
Garnish medallions and sauce as desired with marjoram and halved cherry tomatoes. Sprinkle mashed potatoes with parsley. Serve on plates. Serve with cucumber salad extra.