Before starting this Boeuf en Croûte recipe, make sure you have organised all the necessary ingredients.
Remove the strings and bard from the beef fillet. Sear in a frying pan with a little olive oil.
Sear the beef piece on all sides until golden. Transfer the meat onto a paper towel.
Clean the button mushrooms and slice.
Sauté in a frying pan with a knob of butter and half of the sliced shallots.
Away from the heat, add the sliced truffle. Reserve the juice.
Roll out the puff pastry to a thickness of 3-4mm.
Spread the cold mushrooms on the pastry.
Place the beef fillet on top of the mushrooms. Season with salt and pepper.
Fold the pastry over the meat on two opposite sides and glaze with the beaten egg yolk.
Fold back the two remaining sides of the pastry...
... so that the meat is entirely wrapped. Glaze the edges with egg yolk.
Flip upside down on a baking tray.
Glaze the surface with egg yolk.
If you have scraps of pastry left, use a pastry cutter to make the desired shapes...
... and stick them on the surface.
Glaze the pastry shapes as well.
Bake at 200°C (gas 6) for 20 minutes. Depending on how done you like your meat, the cooking time varies: reduce it if you'd like your meat medium-rare or increase it if you wish to enjoy your beef well done. The indicated 20 minutes is for medium.
While the beef is cooking, wash the wild mushrooms under running water. Sauté in a frying pan with a little olive oil. Set aside.
In a non-stick frying pan, sweat the remaining shallots with butter.
Moisten with brown veal stock.
Add the truffle juice. (The juice can be replaced with truffle flavouring.) Reduce a little...
... then pass the sauce through a chinois or a fine sieve...
... and add the butter, then whisk to give the sauce a creamier consistency. Adjust the seasoning if needed.
Add a few truffle shavings, finely chopped. Keep the sauce warm over a bain-marie.
When the cooking is complete, remove the beef from the oven. Place the beef fillet on a serving dish, with the sautéed wild mushrooms on the side. Serve the sauce separately in a sauce boat.