Rinse the prime rib roast under cold water and pat it completely dry with paper towels.
Place sliced onions across the bottom of a roasting pan or on a rack, then set the roast on top.
Mix salt, pepper, garlic, rosemary, thyme, and olive oil in a bowl until combined into a paste. Rub this mixture evenly over the entire surface of the roast.
Cover the roast and refrigerate it overnight so the flavors can develop.
Remove the roast from the refrigerator a couple of hours before cooking so it comes closer to room temperature.
Preheat the oven to a high temperature and place the roast on a lower rack to prevent excessive browning.
Roast initially at high heat to develop a crust, adding a small amount of water to the pan to prevent burning.
Rotate the roast as needed to ensure even browning on all sides.
Lower the oven temperature and continue roasting, basting occasionally and adding water if the pan becomes dry.
Monitor the internal temperature carefully and remove the roast once it reaches your desired doneness.
Let the roast rest under foil before slicing, allowing the juices to redistribute.
Strain the pan juices and warm them to serve alongside the sliced prime rib.