Mix all the ingredients for the pastry dough and leave to rest for several hours. Roll out the dough to the right size, but at least 5mm thick.
Fry the onions and mushrooms in butter, add parsley, and thicken the duxelles if necessary. Leave the duxelles to cool, spread it out thinly over the pastry, and leave to rest. Sear the veal tenderloin and place it on the duxelles.
Fold the pastry around the tenderloin and ensure the seam is at the bottom during cooking. Glaze with the egg yolk, decorate with the leftovers from the dough, and bake in the oven on 180°C (356°F)until it reaches a core temperature of 56°C (132°F).
* Duxelles is a mix of very finely chopped mushrooms, onion, or shallot and herbs.
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