The familiar pairing of creamy mashed potatoes and succulent lamb takes an unexpected hint of sweet sharpness from its supporting cast of stewed root vegetables.
Preheat oven to 300 degrees. Heat oil in a large Dutch oven over medium-high heat. Season lamb generously with salt and pepper. Working in batches, cook until golden brown, 4 to 5 minutes per side. Transfer to a plate. Reduce heat to medium. Add onion and garlic to pot, and cook until softened, about 4 minutes. Stir in tomato paste and cook for 2 minutes. Stir in wine, bring to a boil, and cook for 1 minute.
Add browned lamb, stock, bay leaf, and thyme to pot. Bring to a simmer; cover, and transfer to oven. Cook for 2 hours. Add parsnips, celery root, and cipollini, and cook until vegetables and meat are tender, about 1 hour.
Meanwhile, bring potatoes to a boil in a pot of salted water. Reduce heat, and simmer until tender, about 30 minutes. Drain, and let cool slightly. Press hot potatoes through a ricer into pot. Add cream, butter, and 2 teaspoons salt. Cook over medium heat, stirring often, until warmed through. (Alternatively, whip potatoes with a mixer until smooth.) Press a piece of plastic wrap directly on surface.
Using a slotted spoon, transfer lamb to a dish, reserving sauce. Remove meat from bones, shredding it into bite-size pieces. Skim fat from sauce. Discard bay leaf and thyme. Return meat to sauce in pot, and add peas. Season with salt and pepper.
Preheat broiler. Spoon lamb into a 6-cup baking dish adding just enough sauce to moisten; reserve remaining sauce. Spread potatoes over lamb. Alternatively, spoon potatoes into a pastry bag fitted with a 3/4-inch plain round tip, and pipe on top of lamb, covering surface. Broil until golden brown, about 5 minutes. Serve immediately with reserved sauce.