For the farce, peel and finely dice the onions. Clean and finely chop the mushrooms. Fry in 1 tablespoon butter. Season with salt and pepper and deglaze with sherry. Cook for about 5 minutes at high heat. Chop parsley finely and stir in. Let the farce cool down
In the meantime, place puff pastry sheets next to each other and allow to thaw. Fry the fillet of beef in a pan with hot oil all around for about 20 minutes. Season with salt and pepper
Place 5 pastry sheets on top of each other. Roll out to a square (30x30 cm) on a floured work surface. Sprinkle the front wide edge with breadcrumbs and add some farce. Place the fillet on top. Put the rest of the mince on the fillet and press it on. Whip in the dough, press on, brush with whisked egg yolk
Roll out the rest of the dough slightly and cut into strips. Cut out 2 mushrooms as desired. Decorate fillet with strips, spread again. Place on a baking tray moistened with water. Bake in a preheated oven (electric cooker: 200 °C/ convection oven: approx. 175 °C/ gas: level 3) for approx. 25 minutes (add puff pastry mushrooms after approx. 10 minutes). Let roast rest for approx. 10 minutes before cutting
Cut kohlrabi into slices, sugar snap peas into strips. Cook kohlrabi for about 10 minutes, sugar snap peas for 5 minutes in boiling salted water
Heat up 1/4 litre of water. Stir in all the sauce powder, bring to the boil. Beat the rest of the butter in small pieces. Chop chervil, add. Refine with cream. Season with salt and pepper. Cut the roast open. Serve with vegetables and sauce. Serve garnished with chervil