Preheat broiler. Trim lamb of any excessive deposits of fat, leaving a thin layer behind. Place in a roasting pan and broil, turning occasionally, until fat crackles and begins to brown on all sides. Remove lamb to a rimmed baking sheet. Mix together minced rosemary and crushed garlic, and spread evenly over roast; sprinkle with salt and pepper and set aside. Reduce oven temperature to 275°F. 

Skim and discard as much fat as possible from the baking pan, taking care not to discard browned bits or meaty juices. Place roasting pan over medium-high heat and deglaze with a bit of white wine, scraping up any browned bits; add remaining wine, bring to a boil, and simmer until reduced by half. Add beef stock and whisk in tomato paste. Crush tomatoes by hand, discarding any skin and tough cores, and add to pan along with any tomato juice. Add bay leaves, rosemary, and garlic and bring to a simmer. Return lamb to pan and distribute onions, carrots, and potatoes around it. Cover the pan tightly with heavy-duty foil, transfer to the oven and roast, turning occasionally, until meat is very tender and falling apart, 5 to 6 hours.

Using a large spatula or slotted spoon, transfer lamb and vegetables to a serving platter; tent with foil.

Transfer pan juices to a gravy separator and allow to settle; pour juices into a large heavy saucepan, leaving as much fat behind as possible. Place over medium heat and bring to a simmer; taste and adjust seasonings. If desired, thicken with a little cornstarch or arrowroot slurry. Transfer to a gravy boat and serve with lamb and vegetables.