Clean the fillet well.

Brown it in a pan for about 2 minutes per side. Transfer the fillet to a plate and brush with mustard.

Cut the previously cleaned champignons into pieces. Blend for a few seconds, then transfer to a pan with a drizzle of extra virgin olive oil and a garlic clove. Dry the mushrooms at moderate heat for about 10 minutes.

Add salt and pepper to taste.

Lay out 2 sheets of plastic wrap on the work surface, place 8 slices of prosciutto to form a rectangle, and spread the mushroom mixture over the prosciutto.

Place the fillet on one of the two ends and, using the plastic wrap, roll the fillet tightly.

Refrigerate for at least 30 minutes.

Lay the rectangular puff pastry on the work surface, place the fillet on top, and roll it in the puff pastry.

You can decorate the Wellington fillet by making light incisions in the puff pastry or using a second sheet of puff pastry, and then brush it with some egg yolk. 

Bake in a static oven at 200°C for about 35/40 minutes.

Remove from the oven and serve with a side of your choice.