Preheat your smoker to 225°F (107°C). If you don’t have a smoker, you can use a grill with a smoking box.

In a small bowl, combine the smoked pasilla chili powder, crushed toasted coriander seeds, kosher salt, black pepper, minced garlic, olive oil, chopped rosemary, and chopped thyme. Mix to form a paste.

Rub the spice mixture all over the prime rib roast, ensuring it is evenly coated. Allow the roast to sit at room temperature for about 30 minutes to absorb the flavors.

While the roast is resting, prepare the caramelized onion compound butter. In a skillet over medium heat, melt 1 tablespoon of butter. Add the sliced onions and cook, stirring occasionally, for about 20-25 minutes or until the onions are golden brown and caramelized.

In a mixing bowl, combine the remaining softened butter, caramelized onions, Worcestershire sauce, and a pinch of salt. Mix until well blended. Transfer the compound butter to a piece of parchment paper, roll it into a log, and refrigerate until firm.

Place the seasoned prime rib roast in the smoker and cook for approximately 3-4 hours, or until the internal temperature reaches your desired doneness (125°F/52°C for rare, 135°F/57°C for medium-rare, 145°F/63°C for medium).

Once cooked, remove the roast from the smoker and let it rest for at least 20 minutes before slicing. This allows the juices to redistribute within the meat.

Serve the prime rib slices with a generous dollop of caramelized onion compound butter on top.