Traditional Shepherd's Pie

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.