Combine 1 cup sour cream, ½ cup mayonnaise, 2 tablespoons horseradish, 1 teaspoon vinegar, and ½ teaspoon salt in mixing bowl. Stir until smooth.

Cover and refrigerate at least 1 hour before serving.

Place 1 whole beef tenderloin on sheet pan with wire rack. Rub entire surface with 1 to 2 tablespoons oil.

Sprinkle 2 tablespoons seasoning blend liberally over entire tenderloin, covering all sides.

Place seasoned tenderloin in refrigerator uncovered for 12 hours or overnight.

Remove tenderloin from refrigerator 30 minutes before smoking.

Fold thin tail end under main body of roast. Secure with kitchen twine.

Continue tying entire tenderloin with kitchen twine at 2-inch intervals along length.

Preheat smoker to 225℉ and add wood chips.

Insert probe thermometer into center of thickest part of tenderloin.

Place tenderloin on smoker grates and close lid. Smoke about 1 hour or until internal temperature reaches 115 to 120℉ for medium-rare.

Preheat cast iron skillet over medium-high heat until very hot.

Add 2 tablespoons butter to hot skillet.

Place smoked tenderloin in skillet and sear 60 to 90 seconds on each side, rotating to sear all sides evenly, until deep golden-brown crust forms.

Remove tenderloin from skillet and transfer to cutting board.

Let tenderloin rest 15 minutes before slicing.

Remove kitchen twine and slice tenderloin into ½ to ¾-inch thick medallions.

Serve immediately with chilled horseradish sauce.