1/2 cup plus 2 tablespoons crème fraîche or sour cream (about 5 ounces)
1 1/2 tablespoons (or more) drained prepared horseradish
1 1/2 tablespoons drained green peppercorns, minced, or coarsely ground pink peppercorns
3 garlic cloves, chopped
1 tablespoon chopped fresh thyme
2 1/2 teaspoons kosher salt
1 1/2 teaspoons freshly ground black pepper
1 1/2 teaspoons freshly ground pink peppercorns
1 3-pound center-cut beef tenderloin at room temperature
1 bunch fresh rosemary
1 tablespoon olive oil
1 tablespoon unsalted butter
Whisk crème fraîche (or sour cream) in a medium bowl until thickened and soft peaks start to form, 1-2 minutes. Fold in horseradish and peppercorns; season with salt and more horseradish, if desired. DO AHEAD Sauce can be made 3 days ahead. Cover; chill.
Combine first 5 ingredients and rub all over meat, pressing to adhere. DO AHEAD Beef can be seasoned 1 day ahead. Wrap in plastic; chill. Let stand at room temperature for 1 hour before continuing.
Preheat oven to 400°. Scatter rosemary over bottom of a large roasting pan.
Heat oil and butter in a 12 inches cast-iron skillet over medium-high heat. Sear meat until brown on all sides, about 5 minutes. Transfer to prepared roasting pan.
Roast meat until an instant-read thermometer inserted into thickest part of beef registers 125° for medium-rare, about 30 minutes. Transfer to a carving board. Let rest for 15 minutes. Thinly slice tenderloin. Serve with Horseradish Cream.
To double or triple the recipe, sear the tenderloins one at a time, then arrange crosswise in roasting pan (cook time will be the same). Leave at least 1 inch-2 inches between pieces of meat.