- 1 well-trimmed 8- to 9-pound rib roast (with 4 bones)
- 6 1/2 teaspoons crushed dried rosemary, divided
- 1 tablespoon coarse kosher salt
- 2 teaspoons freshly ground black pepper
- 1/2 cup plus 2 tablespoons olive oil
- 2 cups boiling water
- 1 1/2 ounces dried porcini mushrooms
- 4 tablespoons (1/2 stick) butter, room temperature, divided
- 10 ounces fresh button mushrooms, cut into 1/4-inch-thick slices
- 4 garlic cloves, minced
- 2 tablespoons all purpose flour
- 1 1/2 cups low-salt beef broth
- 3/4 cup dry red wine
- 2 bunches fresh rosemary (for garnish)
- Place rib roast, fat side up, in heavy roasting pan. Mix 4 teaspoons crushed rosemary, salt, and pepper in small bowl. Stir in 1/2 cup oil. Rub mixture all over roast. DO AHEAD Can be made 1 day ahead. Cover and chill. Let stand at room temperature 45 minutes before roasting.
- Combine 2 cups boiling water and porcini; let soak until mushrooms soften, about 20 minutes. Strain soaking liquid through fine strainer set over measuring cup; reserve porcini. If needed, add enough water to soaking liquid to measure 1 cup. Coarsely chop porcini.
- Melt 2 tablespoons butter with remaining 2 tablespoons oil in large skillet over medium-high heat. Add fresh mushrooms; sauté until browned, about 6 minutes. Add porcini and garlic; stir 1 minute. Remove from heat. Combine flour, remaining 2 tablespoons butter, and remaining 2 1/2 teaspoons crushed rosemary in small bowl; mash with fork to smooth paste. DO AHEAD Porcini soaking liquid, mushroom mixture, and flour-butter mixture can be made 1 day ahead. Cover separately; chill.
- Place rack in bottom third of oven and preheat to 350°F. Cook roast until instant-read thermometer inserted straight down into top center of roast registers 125°F to 130°F for medium-rare, about 2 hours 30 minutes.
- Transfer roast to platter; cover loosely with foil and let rest 35 to 40 minutes. Skim any fat from top of pan juices (there will be a small amount of pan drippings); reserve juices in pan.
- Set roasting pan atop 2 burners over medium-high heat. Add reserved 1 cup porcini soaking liquid, broth, and wine; bring to boil, scraping up any browned bits. Add mushroom mixture, then butter-flour mixture; whisk constantly over medium-high heat until sauce thickens, about 2 minutes. Season sauce to taste with salt and pepper.
- Garnish roast on platter with fresh rosemary sprigs. Slice roast and serve, passing mushroom sauce separately.