In a bowl, combine the softened butter, parsley, tarragon, chives, lemon zest, and lemon juice. Mix well until fully incorporated. Transfer the herb butter onto a piece of plastic wrap. Roll it into a log shape and refrigerate until firm.

Preheat your oven to 450°F (232°C). Pat the prime rib roast dry with paper towels. Rub the roast all over with vegetable oil, then season generously with kosher salt, black pepper, cracked allspice, garlic powder, onion powder, dried thyme, and smoked paprika. Make sure to coat it evenly.

Place the seasoned prime rib on a rack in a roasting pan. Roast in the preheated oven for 15 minutes to develop a crust. After 15 minutes, reduce the oven temperature to 325°F (163°C) and continue to roast until the internal temperature reaches your desired doneness (about 120°F/49°C for rare, 130°F/54°C for medium-rare, 140°F/60°C for medium).

While the prime rib is roasting, prepare the leeks. Heat a grill or grill pan over medium-high heat. Brush the cut sides of the leeks with a little oil and season with salt and pepper. Grill the leeks cut-side down until charred and tender, about 4-5 minutes. Remove and set aside.

Once the prime rib reaches the desired temperature, remove it from the oven, tent it loosely with foil, and let it rest for at least 20-30 minutes. This allows the juices to redistribute.

Slice the prime rib and serve it with the charred leeks on the side. Top each serving with a slice of the herb confit butter, allowing it to melt over the warm meat. Enjoy!