1. Leave roast at room temperature for 1 1/2 hours. Place oven rack in lowest position. Heat oven to 450°F.
2. Set meat, bones down, in a roasting pan. Score fat in 1⁄2-inch crosshatch pattern. In a small bowl, mix olive oil, 1 tablespoon salt, pepper, garlic powder, and paprika; rub all over the roast. Cook 20 minutes.
3. Reduce to 350°F. Roast 60 minutes longer. Carefully add 2 cups water to pan and roast 30–45 minutes longer. Meat is done when an instant-read thermometer inserted in the center of the roast registers 120°F (for medium rare). If you prefer medium, cook roast to 130°F.
4. Transfer roast to a cutting board with a juice groove, cover loosely with foil, and let rest for 20–30 minutes; strain drippings into serving bowl. Cover and keep warm.
5. Meanwhile, melt butter in large skillet over medium-high heat. Add onions and 1⁄4 teaspoon salt. Cook 5–7 minutes, stirring often, until onions are browned. Add mush- rooms and cook 5–7 minutes, until mushrooms release liquid, liquid evaporates, and mushrooms soften and brown. Add 1⁄2 cup water to deglaze. Simmer 2–3 minutes, scraping up browned bits. Remove from heat, cover, and keep warm.
6. Set roast on its side and run a long, sharp knife between bones and meat; remove bones. Turn roast right side up. Carve and transfer slices to platter, top with parsley if desired. Serve with vegetables and jus. 

TIP:
Mushrooms and onions can be prepared up to 1 day in advance. Cover and refrigerate. To reheat, place in small saucepan with 1⁄2 cup water and gently stir over medium-low heat until steaming