Have your butcher cut the ribs from the roast and tie them back on. Use muffin tins if you don't have popover pans. Preheating the pans makes the popover batter start cooking and fluffing the second you pour it in.
Let roast stand at room temperature 1 hour.
Preheat oven to 250°.
Coat a large roasting pan and wire rack with cooking spray. Sprinkle roast with 1 1/2 teaspoons salt and pepper. Heat a large skillet over medium-high heat. Add roast to pan, fat side down; cook 10 minutes, turning roast to brown on all sides. Remove roast from pan; let stand 5 minutes. Discard fat from pan (do not clean pan).
Combine mustard, rosemary, thyme, and garlic in a small bowl. Rub mustard mixture evenly over roast. Place roast, fat side up, on wire rack in prepared roasting pan. Place pan in oven; bake at 250° for 2 hours and 40 minutes or until a thermometer registers 135° (medium-rare) or until desired degree of doneness. Remove roast from oven; let stand 30 minutes. Reserve 2 tablespoons beef drippings.
Sprinkle 1 1/2 teaspoons flour into skillet. Place over medium heat; cook 2 minutes, stirring constantly and scraping pan to loosen browned bits. Stir in wine; cook 2 minutes or until liquid thickens, scraping pan to loosen remaining browned bits. Stir in stock. Increase heat to medium-high. Bring to a boil; cook until liquid is reduced to 1/2 cup (about 8 minutes). Strain stock mixture through a fine sieve over a bowl; return stock mixture to pan.
Preheat oven to 400°.
Coat each of 12 popover cups lightly with cooking spray (omit spray if using nonstick popover pans). Place 1/2 teaspoon of reserved beef drippings into each cup, and place pans in oven for 7 minutes.
Combine milk, egg whites, and whole eggs in a medium bowl, stirring well with a whisk. Weigh or lightly spoon 9 ounces flour into a dry measuring cup. Combine 9 ounces flour and remaining 1 teaspoon salt in a large bowl. Add milk mixture to flour mixture, stirring until combined. Remove pans from oven. Place about 1/3 cup batter in each popover cup. Return pans to oven; bake at 400° for 28 minutes or until popovers puff and turn golden brown.
Cut twine from roast; remove and discard bones. Cut roast crosswise into 12 thin slices. Add beef juices from cutting board to pan with stock mixture. Place pan over medium heat; bring to a simmer. Remove pan from heat; add butter, stirring until butter melts. Serve jus with beef and popovers.
A five lb roast feeding 12 people is a bit of a stretch. The roast is great.