Remove your roast from all its packaging and let it sit out at room temperature for one to two hours. Also make sure your roast is fully thawed, you do not want to cook a roast from frozen. Using paper towels, pat the roast completely dry.
Preheat your oven to 450 F degrees for at least 30 minutes, while the prime rib comes to room temperature.
In a bowl mix the butter with the chili powder, cumin, thyme, rosemary, garlic, salt, and pepper until well combined.
With either a spatula or your hands, spread the compound butter mixture over the entire roast.
Place the onions, garlic, thyme and rosemary in a large skillet that will fit your roast. If you don't have a large enough skillet, use a roasting pan.
Place the roast over the onions in the skillet.
Cook the roast for 15 minutes at 450 F degrees, then reduce the oven temperature to 325 degrees. Continue to cook the roast until your meat thermometer reads 120 degrees. Estimate about 15 minutes of cooking time per pound of prime rib.
Once the thermometer hits 120 F degrees, remove the roast from the oven and transfer it to a cutting board. Cover it with aluminum foil and let it rest for 20 minutes. The roast will continue to cook as the juices inside settle, raising the internal temperature to 130 degrees for a perfect medium-rare prime rib.
Slice and serve with gravy over mashed potatoes.