Horseradish-Crusted Beef Tenderloin With Cream Sauce

1. Sprinkle roast with 1 tablespoon salt, cover with plastic wrap, and let stand at room temperature for 1 hour or refrigerate for up to 24 hours. Adjust oven rack to middle position and heat oven to 400 degrees.
2. Toss panko with 2 teaspoons oil, 1⁄4 teaspoon salt, and 1⁄4 teaspoon pepper in 10-inch nonstick skillet. Cook over medium heat, stirring frequently, until deep golden brown, 3 to 5 minutes. Transfer to rimmed baking sheet and cool to room temperature (wipe out skillet). Once cool, toss bread crumbs with 2 tablespoons horseradish, parsley, shallot, garlic, and thyme.
3. Rinse grated potato under cold water, then squeeze dry in kitchen towel. Transfer potato and remaining 1 cup oil to 10-inch nonstick skillet. Cook over high heat, stirring frequently, until potato is golden brown and crisp, 6 to 8 minutes. Using slotted spoon, transfer potato to paper towel–lined plate and season with salt to taste; cool for 5 minutes. Reserve 1 tablespoon oil from skillet and discard remainder. Once potato is cool, transfer to 1-quart zipper-lock bag and crush until coarsely ground. Transfer potato to baking sheet with bread-crumb mixture and toss to combine.
4. Pat tenderloin dry with paper towels and sprinkle evenly with 1 teaspoon pepper. Heat reserved 1 tablespoon oil in 12-inch nonstick skillet over medium-high heat until just smoking. Sear tenderloin until well browned on all sides, 5 to 7 minutes. Transfer to wire rack set in rimmed baking sheet and let rest for 10 minutes.
5. Combine remaining 2 tablespoons horseradish, mayonnaise, and mustard in small bowl. Just before coating tenderloin, add gelatin and stir to combine. Spread horseradish paste on top and sides of meat, leaving bottom and ends bare. Roll coated sides of tenderloin in bread-crumb mixture, pressing gently so crumbs adhere in even layer that just covers horseradish paste; pat off any excess.
6. Return tenderloin to wire rack. Roast until meat registers 120 to 125 degrees (for medium-rare), 25 to 30 minutes.
7. Transfer roast to carving board and let rest for 20 minutes. Carefully cut meat into 1⁄2-inch-thick slices and serve.
   TO MAKE AHEAD: To make recipe 1 day in advance, prepare through step 3, but in step 2 do not toss bread crumbs with other ingredients until ready to sear meat.

Add the gelatin to the horseradish paste at the last moment, or the mixture will become unspreadable. If desired, serve the roast with HORSERADISH CREAM SAUCE (you will need 2 jars of prepared horseradish for both the roast and sauce). If you choose to salt the tenderloin in advance, remove it from the refrigerator 1 hour before cooking.