Prep Beef Tenderloin: Season tenderloin with kosher salt and ground black pepper. Tie beef tenderloin with kitchen twine (see video tutorial here or photo tutorial above within body of post).  Let come to room temperature (approx. 1 hour).

Prep Smoker: Ensure grill grates are clean and preheat smoker to 225 degrees F. 

Smoke Tenderloin: Place tenderloin directly on the smoker. Close lid and let smoke until the internal temperature reaches approx. 122 degrees F at the thickest part of the tenderloin. Use a dependable internal meat thermometer or probe to track your internal temp. 

Sauce Prep: Meanwhile, combine the Herbed Butter ingredients in a small bowl; stir to combine and set aside. Combine the Horseradish Sauce ingredients in a small bowl; stir and place in the refrigerator. 

Sear: Bring a large cast iron skillet to medium-high heat. Add butter and let melt. Add thyme and move around pan. Sear smoked beef tenderloin on each side for 2-3 minutes or until the internal temp reaches 127 degrees F. 

Rest + Serve: Remove from cast iron skillet and let rest 10-15 minutes. The internal temperature will continue to rise while resting. (The final temperature of a medium-rare beef tenderloin is 130-135 degrees F). Slice and serve with herbed butter, horseradish sauce or creamy mushroom sauce, candied carrots and potatoes au gratin gruyere.