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)