Preheat oven (electric cooker: 160 °C/circulating air: 180 °C/gas: see manufacturer). Pat meat dry and cut into 8 medallions. Dice pancetta. Clean mushrooms and if necessary wash and halve them. Peel and finely dice onions.
Heat the oil in a large frying pan. Fry the medallions in it for 1-2 minutes on each side. Take them out, put them on a baking tray and roast them in the hot oven for about 12 minutes.
Fry the pancetta crisply in the frying fat, remove and drain on kitchen paper. Fry the mushrooms and onions in the bacon fat. Season with salt and pepper. Add 1⁄8 l water and cream, bring to the boil and simmer for about 5 minutes.
Finely grate the cheese. Mix with egg yolk. Wash oregano, shake dry, pluck off the leaves. Remove the mushroom sauce from the stove. Mix a little sauce and egg-cheese mix, then stir everything into the rest of the sauce. Remove meat from the oven, season with salt and pepper. Add the meat to the sauce, possibly with the meat juice, and sprinkle with pancetta and oregano. Serve with toast and salad.