Cut the meat into bite-sized strips. Salt, pepper, dust with flour and mix well. Finely dice the onion and slice the cleaned mushrooms.
In a pan as large as possible, heat some butter as well as oil and sear the meat in several tranches on both sides briefly but vigorously. (If too much meat is fried at once, too much liquid will come out.) The meat should still be pink inside.
Lift each out, place in an ovenproof dish and keep warm, preferably in a slightly preheated oven (to approx. 50 °C).
Pour off excess fat from the pan, heat some fresh butter and first sauté the onions until light. Then add mushrooms and fry everything until there is hardly any liquid left.
Season with salt and pepper, deglaze with a dash of white wine, boil down and add a little beef broth. Mix in whipped cream and let it boil down briefly.
Stir crème fraîche or sour cream with a little flour (approx. 1 KL) until smooth, add and boil down the sauce to the desired consistency.
Add meat kept warm (with juice), heat briefly over low heat, but do not let it boil again, otherwise the meat will become hard.
Arrange Zürcher Geschnetzeltes on preheated plates. Serve with any side dish.