Step 1: Prepare the Beef
Preheat your oven to 275°F and line a sheet pan with aluminum foil. Pat the beef dry with paper towels and brush it generously all over with melted butter. Sprinkle with 4 teaspoons kosher salt and 2 teaspoons freshly ground black pepper—it may seem like a lot, but trust me, it’s worth it.

Place the beef on the prepared sheet pan and roast for 1 to 1¼ hours, until an instant-read thermometer inserted into the thickest part registers 130°F for medium-rare.

Step 2: Add the Dijon Touch
At the 40-minute mark, remove the tenderloin from the oven and brush it with 3 tablespoons of Dijon mustard. Return to the oven to finish roasting.

Step 3: Rest the Beef
Once done, remove the filet from the oven, cover tightly with aluminum foil, and let it rest for 15 minutes at room temperature. This step locks in all those glorious juices. Discard the foil and let it sit uncovered at room temperature.

Step 4: Make the Sauce
In a small bowl, whisk together the mayonnaise, sour cream, Dijon mustard, whole-grain mustard, horseradish, and ¼ teaspoon kosher salt. Refrigerate until ready to serve.

Step 5: Slice and Serve
Remove the strings from the tenderloin and slice it into pieces about ¼ to ½ inch thick. Sprinkle lightly with salt for that finishing touch. Arrange the slices on a platter, garnish with fresh parsley, and serve warm, at room temperature, or cold, with the sauce on the side.