Cut the fillet into wafer-thin slices, dust lightly with flour. Fry in portions in a large pan, remove. Sauté onion cubes in hot fat, add mushrooms, fry for 5 minutes, remove.
Whisk 2 tablespoon cream with egg yolk. Deglaze the roast with the rest of the cream and bring to the boil. Stir the egg yolk cream into the no longer boiling sauce, season with salt, pepper and lemon juice.
Fold in the meat and mushrooms, place in an ovenproof dish, crumble the toast into small pieces, soak lightly with sherry. Spread on the shredded meat, bake in a preheated oven at 200 ° C for 20 minutes.