Place the meat bone side down on a rimmed baking sheet and sprinkle on all sides with the salt and pepper. Let stand at room temperature for at least 1 hour prior to cooking.

Make sure your oven racks are positioned such that the rib roast can fit comfortably. Preheat the oven to 450°F. In a medium bowl, stir together the mustard, garlic, olive oil, lemon juice, and rosemary (reserve the juiced lemon for roasting if desired). Spread the mustard mixture all over the meat.

In a large cast-iron skillet or a roasting pan, place the onions (if you like, you can add the juiced lemon half, as well as the unused lemon half if you have it). Place the meat on top, bone side down.

Roast for 20 minutes at 450°F, then reduce the oven temperature to 350°F. Continue roasting until the meat registers 115°F on an instant-read thermometer inserted as close as possible to the center of the meat without touching bone (or 125°F for medium-rare), about 45 minutes to 1 hour more. Prime rib can overcook quickly, so check early and often. You want it to be nice and pink.

Remove the pan from the oven. Transfer the meat to a cutting board, tent with foil, and let rest for at least 20 minutes before slicing and serving with the pan juices and onions.