How to Make It
Preheat oven to 350°F. Bring 1 1/2 cups water to a boil in a small saucepan over high. Remove from heat, and stir in mushrooms. Cover and let stand until soft and pliable, about 30 minutes. Strain mushrooms through cheesecloth; finely chop mushrooms (reserve liquid for another use).
Rub roast with salt and 1/2 teaspoon of the pepper. Let stand at room temperature 30 minutes.
Stir together chopped mushrooms, butter, shallot, lemon zest, and remaining 1 teaspoon pepper until incorporated. Set aside.
Heat a very large cast-iron skillet over medium-high until hot, about 5 minutes. Add oil, and swirl to coat. Add roast; cook, undisturbed, until a deep brown, about 5 minutes. Flip, and cook until other side is lightly browned, about 2 minutes. Transfer to a baking sheet, and cook in preheated oven until a thermometer inserted in the thickest part registers 125°F, 25 to 30 minutes. Remove from oven, and let stand 10 to 15 minutes. Transfer to a cutting board, and thinly slice against the grain. Serve with porcini-shallot butter.