Serves 4 to 6
Set an oven rack to the lowest position and preheat oven to 400 degrees.
Cut onions into eighths lengthwise and place in a roasting pan just large enough to fit the prime rib. Drizzle with 1 tablespoon olive oil and toss to coat evenly.
Secure outer layer of meat by tying pieces of twine parallel to and between the ribs; this will ensure even cooking. Drizzle 1 tablespoon olive oil over meat and rub surface to coat evenly. Sprinkle salt, pepper, thyme, and garlic powder evenly on all sides.
Heat a large saute pan with remaining 1/2 tablespoon olive oil over medium-high heat. Add prime rib and cook all sides of meaty part until golden brown, about 2 minutes per side. Place prime rib in prepared roasting pan on top of onions. Set aside until cooled to room temperature. (This will help ensure even cooking.) Once meat has reached room temperature, place in oven and cook for 15 minutes. Reduce oven temperature to 325 degrees and cook until meat reaches a temperature of 115 degrees for medium-rare, about 45 minutes longer. Remove from oven, tent with aluminum foil, and let rest for 20 to 30 minutes; the temperature will continue to rise. When cool enough to handle, slice meat from ribs and cut into slices across the grain.