This special-occasion dish takes extra time to prepare. Make the lamb early so the rub has time to fill the meat with flavor.
Trim fat from meat. Set aside.
In a food processor bowl with metal blade, combine dried mushrooms, salt, and black pepper. Cover; process mushrooms to a powder. Add oil, Worcestershire sauce, shallots, and garlic. Cover; process mixture until pureed. Stir in thyme and 2 tablespoons rosemary. Using your fingers, gently press the porcini-herb mixture onto all meaty sides of lamb. Place lamb in a shallow dish. Cover; refrigerate for several hours or overnight.
Remove lamb from refrigerator. Place lamb, meaty side up, on a rack in a shallow roasting pan. Roast in a 325 degree F oven until instant-read thermometer inserted in a meaty portion of lamb registers 140 degree F for medium-rare (1 to 1-1/4 hours). Transfer lamb to a cutting board. Cover loosely with foil; let stand 15 minutes before carving. (The meat's temperature will rise 5 degree F during standing.)
To carve lamb, cut into chops by slicing between rib bones.
To serve, evenly divide the White Bean Ragout in 4 shallow soup plates. Arrange the chops around the piles of beans. Garnish each serving with rosemary sprigs and chopped chives, if you like.