Preheat your oven to 450°F (232°C).

In a small bowl, combine toasted nigella seeds, cracked green peppercorns, kosher salt, and black pepper. Mix well.

Rub the prime rib roast with olive oil and Dijon mustard, ensuring it is evenly coated.

Generously coat the roast with the spice mixture, pressing it onto the surface of the meat.

Place the roast in a large roasting pan, bone side down, and insert a meat thermometer into the thickest part of the roast.

Roast in the preheated oven for 20 minutes to develop a crust. Then, reduce the oven temperature to 325°F (163°C) and continue roasting until the internal temperature reaches your desired doneness (about 130°F for medium-rare, approximately 1.5 to 2 hours).

While the roast is cooking, prepare the charred leek gravy. In a skillet, melt butter over medium heat. Add the sliced leek and sauté until charred and softened, about 5-7 minutes.

Pour in the red wine and scrape up any browned bits from the bottom of the skillet. Let the wine reduce by half.

Add the beef stock to the skillet and bring to a simmer. Cook for an additional 10-15 minutes until the gravy thickens slightly. Season with salt and pepper to taste.

Once the prime rib is cooked to your liking, remove it from the oven and let it rest for at least 20 minutes before slicing.

Slice the prime rib and serve it with charred leek gravy drizzled over the top. Garnish with fresh thyme sprigs.