In a dry skillet over medium heat, toast the fennel seeds until fragrant, about 2-3 minutes. Allow to cool slightly, then grind coarsely using a mortar and pestle or spice grinder.

In a small bowl, combine the smoked ancho chili powder, ground fennel seeds, kosher salt, black pepper, minced garlic, and chopped rosemary.

Preheat your smoker to 225°F (107°C). If you do not have a smoker, preheat your oven to 225°F (107°C).

Rub the prime rib roast with olive oil and then apply the spice mixture evenly over the surface of the roast.

Spread Dijon mustard over the roast for added flavor and moisture.

Place the prime rib roast in the smoker and smoke for about 4-6 hours, or until the internal temperature reaches your desired doneness (125°F for rare, 135°F for medium-rare, 145°F for medium). If using the oven, place the roast in a roasting pan and roast until it reaches the desired internal temperature, basting occasionally with pan juices.

In a skillet, heat olive oil over medium heat. Add the sliced leeks and cook until they are soft and caramelized, about 10-15 minutes.

In a bowl, combine the softened butter, caramelized leeks, fresh thyme, and season with salt and pepper. Mix until well combined and set aside.

Once the prime rib reaches the desired temperature, remove it from the smoker or oven and let it rest for at least 20 minutes before slicing.

Serve the sliced prime rib with a dollop of caramelized leek compound butter on top.