How to Make It
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.