Step 1

Working in batches, use a spice grinder to pulverize the porcini to a fine powder, then transfer it to a small bowl. Repeat with the rosemary and 2½ teaspoons kosher salt, pulverizing until finely ground and adding it to the mushroom powder. Repeat with the peppercorns. Stir the seasonings, then measure 1 tablespoon into another small bowl and set aside. Sprinkle the remaining mix all over the tenderloin, rubbing it in.

Step 2

Tuck the tapered end of the tenderloin under itself so the roast is of an even thickness. Using kitchen twine, tie the tenderloin at 1-inch intervals, making sure the tucked portion is secured with the twine. Brush the roast all over with 3 tablespoons of the oil, place on a V-style roasting rack and set aside at room temperature until ready to roast.

Step 3

Heat the oven to 450°F with a rack in the middle position. In a large (about 12 by 15 inches) roasting pan, toss the portobello mushrooms, onion, remaining 3 tablespoons oil and ½ teaspoon each kosher salt and ground pepper. Roast for 15 minutes. Remove the pan from the oven, stir the mushroom mixture and set the rack with the tenderloin in the pan. Roast until the thickest part registers 120°F for rare or 125°F for medium-rare, 35 to 45 minutes.

Step 4

Transfer the roast to a large platter and remove the rack from the pan. Tent the roast with foil and let rest for 30 minutes. If there is liquid in the pan, before proceeding to the next step, stir the mushroom mixture, return the pan to the oven and cook until the liquid has evaporated.

Step 5

To the mushroom mixture, stir in half of the reserved seasoning mix, the vinegar and parsley. Transfer the rested tenderloin to a cutting board. Remove the twine and carve into ½-inch slices. Pour any juices on the platter and cutting board into the mushroom mixture, then taste the mushrooms and season with kosher salt and pepper. Transfer the mushrooms to the platter, then arrange the sliced tenderloin on top. Sprinkle with the remaining seasoning mix, along with flaky salt (if using).