Mix kosher salt, black pepper, garlic powder, dried thyme, and dried rosemary in a small bowl and rub evenly over all surfaces of the roast.

Place the seasoned roast on a wire rack set over a rimmed baking sheet and refrigerate uncovered for 24 to 48 hours.

Remove the roast from the refrigerator 2 hours before cooking and let it come to room temperature.

Preheat the oven to 225°F and position racks in the middle and lower-middle spots.

Heat a cast iron skillet over medium-high heat until smoking, add vegetable oil, then sear the roast fat-side down for 3 to 4 minutes. Flip and sear the other sides for 2 to 3 minutes each.

Transfer the roast to a roasting pan, insert an instant-read thermometer into the thickest part of the meat, and add smashed garlic cloves and fresh thyme sprigs around the roast.

Roast in the preheated 225°F oven until the internal temperature reaches 115 to 120°F, about 2 to 2.5 hours.

Melt butter in a small saucepan, tilt the roasting pan to collect pan juices, spoon the melted butter over the roast, then remove the roast and tent loosely with foil. Rest for 15 to 20 minutes.

Slice the roast against the grain into 1/2- to 3/4-inch thick slices and serve immediately with pan juices spooned over the top.