Ensure your prime rib roast has a nice layer of fat on top for natural basting. Rinse the roast under cold water, then pat it dry with paper towels to prepare for marinating.

Position a flat roasting rack inside a large roasting pan to accommodate your roast. Spread the onions across the rack. If you don’t have a roasting rack, you can place the roast directly on the onions. Place the roast in the pan.

In a bowl, combine salt, pepper, garlic, rosemary, thyme, and olive oil to create a seasoning blend. Use your hands to thoroughly rub this mixture over the roast, ensuring an even coating across the surface.

Cover the roast with plastic wrap, and let it marinate overnight (at least 8 hours) in the refrigerator. This allows the flavors to penetrate the fat cap, infusing the roast with rich flavors.

Take the roast out of the refrigerator at least 2 hours before cooking to allow it to reach room temperature. For best results with bone-in roasts, consider leaving it out for 6 to 8 hours. Preheat your oven to 450°F (230°C). Position the roast on a lower oven rack, particularly for bone-in roasts; placing it too high may cause premature browning.

Roast for 30 minutes, then add 1/2 cup of water to the bottom of the pan to prevent the onions from burning. This allows them to caramelize and contribute to the roast’s flavor.

Continue roasting for another 30 minutes, checking for even browning. Rotate the roast 180° if necessary, positioning any unbrowned fatty areas toward the oven’s center. Once the outside is well-browned (approximately 1 hour in total), reduce the heat to 350°F (175°C).

Roast for an additional 90 to 120 minutes, basting every 30 minutes if desired. Add 1/2 cup of hot water as needed to prevent the pan from drying out.

A general guideline is to roast for about 15 minutes per pound. For a 12-pound roast, aim for a total cooking time of 180 minutes: 60 minutes at 450°F and 120 minutes at 350°F. These times are approximate; use a meat thermometer to avoid overcooking. For medium-rare, roast until the internal temperature reaches 125°F (51°C). For rarer doneness, remove the roast at 120°F (49°C) or even 115°F (46°C) for very rare. The meat will continue to cook slightly while resting.

If the top begins to char, cover it lightly with foil, leaving the sides exposed. Rotate the roast 180° if necessary to ensure even cooking.

Once done, transfer the prime rib to a cutting board or serving platter. Tent it with aluminum foil and let it rest for 15 to 20 minutes. Meanwhile, strain the pan juices through a fine-mesh strainer into a fat separator. If more liquid is needed, add 1/2 cup of water to the hot pan, stirring to release any flavorful bits.

Pour the separated juices into a small pot or sauté pan and warm them as au jus for serving. Dilute with hot water to taste if the au jus is too concentrated. Serve the prime rib with au jus on the side, and add horseradish if desired.