This prime rib is marinated in a sweet red wine mixture that helps break down the muscles in the meat to make it extra tender. It’s also roasted inside of an oven bag with onions and rosemary so that it retains its bold and strong flavors and stays extra moist.
Mix together the red wine, sugar, garlic, onions, and oil and whisk. Stir well until it is all dissolved.
Pour the marinade over the top of the roast, cover, and store chilled for a minimum of 12 hours, and up to 36 hours.
Remove the roast from refrigerator 1-2 hours before roasting to allow it to come to room temperature. Preheat oven to 375 degrees. Put 1 tablespoon of flour into the oven bag, shake, and then place bag into large roasting pan.
Remove the roast and onions from marinade. Season the roast with salt and pepper. Lay the rosemary sprigs on top of the roast and tie them on using a kitchen twine.
Put the onions in the middle of the roasting bag and place the beef onto the onions. Insert an instant-read thermometer into meat. Seal the bag and put the roast into the oven. Roast for 1 hour 45 minutes, or until the beef reaches an internal temperature of 135 degrees, which is medium-rare; if you want well-done, leave until desired temperature is reached.
Remove the roast from the oven and allow to rest 15-20 minutes before carving. Carve slices across grain of meat, and serve with roasted onions.