The day before, order the brioche dough from the baker.
Heat the butter in a casserole dish and brown the beef tenderloin on each side.
Remove the meat from the casserole, season with salt and pepper and leave to cool.
In the same pot, deglaze the cooking juices with 40 g of butter and the balsamic vinegar. Throw the cleaned and sliced mushrooms into the pan and sauté them for 10 minutes. Salt and pepper.
Roll out the brioche dough with a rolling pin to a thickness of 5 mm.
Lightly butter the baking sheet and place the dough on it.
Arrange the truffle slices on the bottom of the pastry, in the centre, overlapping them slightly.
Place the beef tenderloin on top. Moisten the edges of the pastry with water and fold the pastry over the fillet. Pinch to seal the edges.
Using a thin knife blade, draw a harmonious grid on the dough or a leaf, taking care not to pierce the dough. Brush with beaten egg yolk. Let stand 30 minutes.
Bake the surprise beef for 30 minutes in a preheated oven at 180°/200°C.
Once cooked, turn off the oven and let the tenderloin rest for 10 minutes before serving.
Meanwhile, finish the sauce: pour the port into the pot, salt and pepper. Heat and add the remaining butter, beating the sauce with a whisk.
Place the brioche on a heated serving platter and serve with the sautéed sauce, chanterelle mushrooms and turnips.