Mix together the salt, pepper, onion, garlic, parsley and pepper flakes.
Cooking prime rib isn’t hard when you can slow roast it on a gas grill . Since the beef roast is the star of your meal, start with the best roast you can find. Then all you need is a simple beef rub and Char-Broil’s® TRU-Infrared™ cooking system to create a juicy, delicious roast fit for a king.
Remove the butcher's twine and lightly coat the roast and ribs with oil.
Season on all sides (including the space between the ribs and roast) with the dry rub. Re-tie the beef roast and ribs back together. Let sit at room temperature for an hour.
Set up your grill for indirect heat and preheat to low (250-300°F).
Place the roast, bone-side down, on a roasting rack and pan combination. Add enough stock to fill the pan about 1 inch deep. Add the dried mushrooms.
Place the roast and pan on the grill on the indirect side where you don't have any burners on. Close the grill lid and cook until the rib roast hits 10 °F less than your desired level of doneness. This should take about 3 hours for medium rare. It's a good idea to turn the roast around every 45 minutes or so. (For rare - pull at internal temperature of 115°F. For medium rare – 125°F. For medium – 135°F).
Remove the roast and pan set up and let rest at room temperature until the internal temperature stops rising - about 20 minutes. Meanwhile, turn up the grill heat to high (500 °F).
Taste the au jus (seasoned broth in the pan) and season with salt and pepper to taste. Keep warm.
Fully remove the rib bones. Place the rib bones cut side down over the heat. Sear the roast 2-3 minutes on the cut side and 1-2 minutes on the other sides.
Remove from the grill. You can let it rest for 5 minutes but it isn't needed - it already settled during the previous rest.
Slice, top with some of the porcini au jus and serve.