Trim and tie the tenderloin at least 2 hours before grilling, and ideally the night before. Lay the tenderloin on your cutting board and use a sharp boning knife to remove all silverskin - that tough, silvery connective tissue that will not break down during cooking and creates an unpleasant chewy texture. Fold the thin tail end of the tenderloin back under itself and tie securely so the roast is roughly the same diameter from end to end. Then tie additional loops of butchers twine every 2 inches along the entire length. This step is critical for even cooking - an untied tenderloin is tapered and will overcook on the thin end before the thick center reaches temp.

Season aggressively and rest the tenderloin on the counter. Coat the entire surface with olive oil, then combine the kosher salt, black pepper, garlic powder, smoked paprika, onion powder, crushed dried rosemary, and dried thyme in a small bowl. Rub the seasoning blend all over every surface of the tied tenderloin, pressing it in firmly. Do not be shy here - a roast this size needs bold seasoning to deliver flavor all the way to the crust. Place the seasoned tenderloin on a wire rack set over a sheet pan and let it sit uncovered at room temperature for at least 45 minutes to allow the seasoning to penetrate and the surface moisture to dry out slightly. A dry surface means a better crust.

Fire up the grill for a two-zone indirect setup. If you are using a charcoal kettle, light a full chimney of hardwood charcoal and once the coals are ashed over, bank them all to one side of the grill. This gives you a direct heat zone over the coals and an indirect zone on the opposite side where there are no coals beneath the grate. Toss your hardwood smoking chunks directly onto the coals - cherry or oak wood pairs beautifully with beef tenderloin. Replace the grill grate and let the grill stabilize at 225 to 250 degrees Fahrenheit on the indirect side with the lid closed. This low and slow phase is where the smoke does its work.

Place the tenderloin on the indirect heat zone, away from the coals, and close the lid. Let it cook low and slow at 225 to 250 degrees Fahrenheit, turning once halfway through, until the internal temp at the thickest point reads 115 degrees Fahrenheit on your instant read thermometer. This phase typically takes 35 to 50 minutes depending on the thickness of your roast and your grill temperature. Resist the urge to lift the lid constantly - every time you open it, you lose heat and smoke. At 115 degrees internal, you will notice a faint smoke ring just beneath the surface and a light crust beginning to form. That is your cue to get ready for the sear.

While the tenderloin rests on the indirect side, open all grill vents completely and add a fresh half chimney of lit coals directly on top of the banked coals to get the direct zone scorching hot. You want the grates directly above the coals to reach 500 to 600 degrees Fahrenheit - hold your hand 6 inches above the grate and if you cannot keep it there for even 1 second, the fire is ready. Set a small cast iron skillet on the grill grate near the edge of the fire and add the butter, smashed garlic, fresh rosemary sprigs, and fresh thyme. Let the butter melt and begin to foam while you prepare to sear.

Transfer the tenderloin directly over the hot coals and sear it, turning with long tongs every 60 to 90 seconds, until a deep mahogany crust has formed on all sides - this should take no more than 5 to 7 minutes total. Every 90 seconds, spoon the garlic herb butter generously over the top of the tenderloin as you rotate it. You want color on every surface including the ends. Watch for flare-ups - if the fat from the baste causes a flame-up, use your tongs to move the tenderloin slightly off the hottest part of the coals for a moment, then continue. The internal temp will climb quickly during this phase - check it frequently.

Pull the tenderloin off the grill when the internal temp at the thickest point reaches 125 to 128 degrees Fahrenheit for medium-rare. Remember that carryover cooking will raise the temp another 5 degrees or so as it rests, bringing it to a perfect 130 to 133 degrees Fahrenheit medium-rare. Transfer the tenderloin to your cutting board with a juice groove, tent it loosely with aluminum foil, and rest the meat for a minimum of 10 minutes - 15 minutes is even better. Do not skip the rest. Cutting into it early will release all the juices that are still redistributing through the meat, leaving you with a dry, less flavorful result.

Remove the butchers twine, slice the tenderloin into 1-inch to 1.5-inch medallions with a sharp slicing knife, and arrange them on a warm serving platter or directly on the cutting board. Spoon any remaining garlic herb butter and the collected resting juices over the sliced medallions. Finish with a light pinch of flaky sea salt over each slice and garnish with fresh rosemary sprigs. Serve immediately while the crust is still crackling and the interior is a gorgeous, even rosy pink from edge to edge.