Preheat your oven to 450°F (232°C).

In a dry skillet over medium heat, toast the fennel seeds until fragrant, about 2-3 minutes. Remove from heat and let cool.

Grind the toasted fennel seeds and smoked star anise in a spice grinder or mortar and pestle until fine.

Rub the prime rib roast with olive oil, then generously season with salt and pepper.

Coat the roast with the ground spice mixture, pressing it into the meat to adhere well.

Place the seasoned prime rib in a roasting pan and roast in the preheated oven for 20 minutes.

After 20 minutes, reduce the oven temperature to 325°F (163°C) and continue roasting until the internal temperature reaches your desired doneness (135°F for medium-rare, 145°F for medium), about 1.5 to 2 hours more.

While the roast is cooking, heat a large skillet over medium heat. Add a tablespoon of olive oil and the sliced onions.

Cook the onions, stirring frequently, until they are deep golden brown and caramelized, about 20-25 minutes. Add garlic during the last 5 minutes of cooking.

In a bowl, mix the caramelized onions with softened butter and fresh thyme leaves. Season with salt and pepper to taste. Set aside.

Once the prime rib reaches your desired doneness, remove it from the oven and let it rest for at least 20 minutes before slicing. This allows the juices to redistribute.

Slice the prime rib and serve it warm, topped with a generous dollop of the caramelized onion butter. Garnish with fresh herbs if desired.