DO AHEAD: Pat tenderloin down with a paper towel. Tuck any long thin piece under the roast. Use kitchen twine to tie into an even log, placing the kitchen twine about 1-inch apart. Sprinkle the roast with 1 ½ teaspoon kosher salt and 1 teaspoon black pepper, press to make sure it adheres. Allow the roast to sit in the refrigerator, uncovered, for a minimum of 1 hour or ideally, 8-10 hours. Remove the roast from the refrigerator 1 hour before cooking.

SEAR: Position a rack in the center of the oven + preheat the oven to 300ºF. Place 1 tablespoon oil in a large cast-iron skillet over medium-high heat. Gently place the tenderloin in the skillet and allow it to sear evenly on each side for about 3-4 minutes or until each side gets a golden-brown crust. Remove to a plate.

PAN GRAVY: Lower heat to medium. Add the remaining tablespoon of oil and 2 tablespoons of butter. Add onions and a pinch of salt. Scrape to pick up any stuck-on bits and cook for 2-3 minutes. Add the baby Bella mushrooms and continue to cook for another 6-7 minutes or until mushrooms turn golden brown. You may need to cradle the heat between medium and medium-high to help evaporate the liquid and brown the mushrooms. Sprinkle with half the garlic and half the thyme and saute for 30 seconds. Deglaze pan with vinegar, stir, and simmer until it evaporates. Pour in the prepared stock and allow it to come to a simmer. Add the porcini mushrooms. Simmer for 2 minutes, remove from heat.

SLATHER: In a bowl, combine 2 tablespoons softened butter, dijon, horseradish, remaining minced garlic, and thyme. Brush or rub this all over the tenderloin.

ROAST: Place the tenderloin on a wire rack (see notes) in the center of the prepared pan gravy. Roast until the center of the roast registers between 120-135ºF based on preference. It will take about 40-52 mins for medium-rare and 53-65 mins for medium. You want the temperature of the roast to be about 10 degrees less than your ideal cook (see notes.) Remove the tenderloin from the pan to a cutting board. Let rest 15 minutes, undisturbed.

SERVE: Place the gravy back on the stove over medium heat and cook until it reduces to a gravy-like consistency. Taste + adjust with salt and pepper as desired. Stir 1 tablespoon cold butter into the sauce if you want it to have shine and luster, this is optional. Remove twine and cut meat crosswise into ½ inch thick slices. Serve with prepared pan sauce.