Preheat oven to 400°F/204°C/Gas Mark 6.

Place dried mushrooms into a mini food processor or blender and grind until you have a fine powder. It's ok if there are a larger pieces, you can sift them out.

Pour the mushroom powder into a fine mesh strainer and sift over a bowl to remove the larger bits. Save the large pieces to add to soups or put them back in to the processor to grind.

Fold under the tail end (the thin end) of tenderloin and tie with butcher's twine. Keeping the thickness uniform helps the meat cook evenly. Season with salt and pepper.

In a heavy-bottom sauté pan (skillet), heat the oil until shimmering. Place tenderloin in pan and sear on all 4 sides. About 6 - 8 minutes total.

Dust on all 4 sides with porcini powder, giving it a heavy coating.

Slide into the oven and let it roast for 10 minutes at 400°F, then turn down to 325°F/165°C/Gas Mark 3 for an additional 10-12 minutes.

Remove from oven and place tenderloin on sheet pan to rest. I let it rest for 5 minutes on one side, then flip it over and let it rest an additional 5 minutes.

Cut twine off of roast and slice.