Remove the prime rib from the refrigerator and let it come to room temperature for about 1 hour. Preheat your oven to 450°F (232°C).

In a small bowl, combine the ground star anise, smoked sea salt, black pepper, garlic powder, and onion powder. Rub the olive oil all over the prime rib, then evenly coat the meat with the spice mixture.

Place the seasoned prime rib on a rack in a roasting pan, bone-side down. Roast in the preheated oven for 20 minutes. Then, reduce the oven temperature to 325°F (163°C) and continue roasting until the internal temperature reaches your desired doneness (about 120°F for rare, 130°F for medium-rare, or 140°F for medium). This will take approximately 1.5 to 2 hours.

Once cooked, remove the roast from the oven and let it rest for at least 20 minutes before carving.

While the prime rib is resting, heat a large skillet over medium heat. Add the sliced onions and cook, stirring occasionally, until they are soft and caramelized, about 25-30 minutes. Reduce the heat if the onions start to burn.

In a medium bowl, combine the softened butter, caramelized onions, and fresh thyme leaves. Mix well until fully incorporated. Season with salt to taste.

Carve the prime rib into slices and serve with a dollop of caramelized onion butter on top. Garnish with fresh thyme sprigs.