Preheat your oven to 450°F (232°C). In a small bowl, combine smoked paprika, sea salt, black pepper, garlic powder, and onion powder. Mix well. Rub the spice mixture evenly over the entire surface of the prime rib roast.

Heat a grill or grill pan over medium-high heat. Drizzle the halved green onions with olive oil and season with salt. Grill the green onions for 2-3 minutes on each side, or until charred and tender. Remove from heat and set aside.

Place the seasoned prime rib on a roasting rack in a baking dish. Insert a meat thermometer into the thickest part of the roast. Roast in the preheated oven for 20 minutes, then reduce the temperature to 325°F (163°C) and continue cooking until the internal temperature reaches your desired doneness (about 130°F/54°C for medium-rare). Remove the roast from the oven and let it rest for at least 20-30 minutes before slicing.

While the roast is resting, melt 1/4 cup of the unsalted butter in a skillet over medium heat. Add the chopped leeks and cook until softened and caramelized, about 8-10 minutes. Stir in fresh thyme leaves and cook for an additional minute. Remove from heat and let cool slightly. In a mixing bowl, combine the remaining 1/4 cup of softened butter, the sautéed leeks, Dijon mustard, and whole mustard seeds. Mix until well combined. Set aside.

Slice the rested prime rib and serve with the charred green onions and a generous dollop of roasted leek butter on top.