- To make popovers, preheat oven to 450˚F. Place eggs in a bowl and beat well. Mix in milk and water, and then whisk in all-purpose flour. Now stir in the green onion, 1 Tbsp. of chopped rosemary and salt and pepper, to taste.
- Thoroughly grease a good-quality (scratch-free) non-stick muffin pan with oil spray. Pour in batter almost to the top. Bake popovers in the middle of the oven 10 minutes. Reduce heat to 325˚F and bake 35 to 40 minutes more, or until popovers are puffed and golden and cooked in the centre. When cool enough to handle, carefully remove from tin and reserve (see Note).
- Place roast bone-side down in a roasting pan. Brush the mustard on the top and sides of the roast. Sprinkle with remaining rosemary, salt and pepper.
- Roast the beef 20 minutes, and then reduce oven temperature to 325˚F. Finish cooking roast to desired doneness, allowing about 20 to 25 minutes per pound for rare to medium rare. To gauge doneness, always use an instant-read meat thermometer, inserted into the centre of the thickest part of the roast. For a rare roast, remove from the oven when the internal temperature is 120°F to 125°F; 130°F for medium rare; and 140°F for medium.
- When beef is cooked, transfer to a plate, loosely cover with foil and rest 10 to 15 minutes before thinly slicing and serving with the popovers.
Note: Popovers are another name for Yorkshire pudding cooked in muffin tin. You can make them many hours before needed. Keep them covered once removed from the tin and cooled to room temperature. When needed, set the popovers on a baking sheet and warm them in the oven a few minutes while the prime rib is resting.
Recipe options: To make jus to serve with the roast, when the meat is resting, skim fat from the juices in the roasting pan, and then set pan over medium-high heat. Pour 3 cups of beef stock (or broth) into pan, bring to a simmer, and simmer 5 minutes. Pour jus into a sauceboat and serve with the beef.