Take the veal fillet out of the fridge in good time and let it come to room temperature. Heat 1 tbsp. clarified butter and stir-fry the shallots, mushrooms and garlic for about 5 minutes. Add salt, pepper and cognac. Continue frying until all the liquid has evaporated. Leave to cool and then purée with the parsley. 

Preheat the oven to 180°C (356°F) convection oven. Line a baking tray with baking paper. Heat the remaining clarified butter on high. Sear the veal fillet on all sides for 4 minutes. Remove, leave to cool slightly, then brush with mustard.

Defrost the puff pastry, overlapping it so that it can enclose the fillet. Press down the overlapping edges. If desired, cut off a piece of pastry and use it to make decorations.

Spread about a third of the mushroom duxelles in the centre of the puff pastry so that the lower third of the veal fillet can be wrapped in it. Spread the rest of the duxelles over the meat. Wrap with puff pastry and place seam side down on the tray. Brush decorations as desired with egg white on one side and place on top. Brush the top with egg yolk and insert a kitchen thermometer into the meat. Bake for 10 minutes, then reduce the temperature to 120°C (248°F) and continue cooking the fillet for about 7 minutes until the core temperature is 60°C (140°F) and the puff pastry is golden brown. Serve with a dark sauce made from veal stock or a béarnaise sauce.