For the Filling
- 3 tablespoons safflower oil
- 5 pounds lamb shanks
- Coarse salt and freshly ground pepper
- 1 medium onion, finely chopped
- 3 garlic cloves, minced
- 2 tablespoons tomato paste
- 1/2 cup dry red wine
- 2 1/2 cups chicken stock
- 6 thyme sprigs
- 1 dried bay leaf
- 2 cups celery root, cut into 1/2-inch pieces
- 3 medium carrots, thinly sliced
For the Topping
- 5 Yukon Gold potatoes (about 2 pounds)
- Coarse salt
- 3/4 cup heavy cream
- 1 stick unsalted butter, cut into pieces
- Preheat oven to 300 degrees. Make the filling: Heat oil in a large Dutch oven over medium-high heat. Season lamb shanks generously with salt and pepper. Working in batches, brown lamb shanks on all sides, about 8 minutes. Transfer lamb to a large plate using kitchen tongs. Reduce heat to medium. Add onion and garlic to Dutch oven, and cook until softened, about 4 minutes. Add tomato paste, and cook for 2 minutes. Add wine, and bring to a boil. Cook for 1 minute.
- Return lamb to Dutch oven. Add stock, thyme, and bay leaf. Bring to a simmer. Braise,
covered, for 2 1/2 hours.
- Remove Dutch oven from oven, and add celery root and carrots. Braise until meat is falling off the bone and vegetables are tender, about 30 minutes. Let cool slightly.
- Meanwhile, make the topping: Peel potatoes, and cut into 1-inch cubes. Transfer to a medium pot, and cover with salted water. Bring to a boil. Reduce heat to low, and simmer until tender, about 20 minutes. Drain, and pass potatoes through a ricer. Return potatoes to pot, and add cream, butter, and 2 teaspoons salt. Cook over medium heat, stirring until combined and warmed through. (If not assembling potpies immediately, place a piece of plastic wrap directly on the surface of potatoes to prevent a skin from forming.)
- Transfer lamb to a plate using a slotted spoon, and remove meat from bones. Break into smaller pieces using a fork. Skim fat from remaining cooking liquid. Return lamb to Dutch oven, and stir.
- Heat broiler. Transfer filling to a 12-inch (8-cup) gratin dish using a slotted spoon. Add 1/4 cup cooking liquid. Spread mashed potatoes over top, covering the entire surface. Broil 5 inches from heat source until potatoes are golden brown, about 5 minutes. Serve remaining cooking liquid on the side.
Mashed potatoes can be refrigerated for up to 3 days; reheat before using.