Cook potatoes with their peel in the oven at approx. 175 degrees for approx. 50-60 minutes, then peel them and either strain them through a sieve or press them through the potato press. Let cool down. You can prepare a few hours in advance.
Preheat the oven to 125 degrees. Salt and pepper the pork tenderloin and sear it on all sides in a pan, place on a plate and place in the oven for about 30 minutes. Peel off the roast set in the pan with the gravy, ideally transfer to a small saucepan, bring to the boil, add the peppercorns, add the cognac, cream and creme fraiche, simmer, assemble with cold butter and season to taste.
Separate egg yolk and egg white, whip egg white. Melt the butter, mix the egg yolk, quark and butter with the potatoes, season with salt, pepper and nutmeg. Finally fold in the egg whites.
Heat the oil in a pan, portion the potato mixture with a spoon and shape into cookies in the pan, fry for 2-3 minutes on each side until golden brown.
Cut the pork fillet into thin slices and arrange on plates, cover with the sauce and add the biscuits.