In a skillet over medium heat, toast the fennel seeds and allspice berries until fragrant, about 2-3 minutes. Allow to cool slightly, then coarsely crush them in a mortar and pestle or spice grinder.

Pat the prime rib roast dry with paper towels. Rub the roast all over with olive oil, then season generously with the crushed spices, sea salt, and black pepper. Let the roast sit at room temperature for about 1 hour to allow the flavors to meld.

Preheat your oven to 450°F (232°C).

Place the seasoned prime rib on a rack in a roasting pan, bone side down. Roast in the preheated oven for 20-25 minutes until browned. Reduce the oven temperature to 325°F (163°C) and continue roasting until the internal temperature reaches 130°F (54°C) for medium-rare, about 1.5 to 2 hours. Use a meat thermometer for accuracy.

While the roast is cooking, heat a large skillet over medium heat. Add the sliced leeks and a pinch of salt. Sauté until soft and caramelized, about 15-20 minutes. Remove from heat and set aside.

In a bowl, combine the softened butter with the caramelized leeks. Mix well and season with salt and pepper to taste. Set aside.

Once the prime rib reaches the desired temperature, remove it from the oven and let it rest for at least 20 minutes before slicing. This helps retain the juices.

Slice the prime rib and serve with a generous dollop of caramelized leek butter on top. Garnish with fresh thyme sprigs.