For the Topping

• 6 large russet potatoes, peeled and cut into large pieces (about 2 to 2½ pounds)
• 2 ounces (4 tablespoons) unsalted butter, cut into small cubes
• ¾ cup to 1 cup milk or buttermilk, as needed
• ½ teaspoon freshly grated nutmeg
• 1 large egg yolk
• Salt and white pepper to taste

For the Filling

• 5 tablespoons extra virgin olive oil, divided
• Kosher salt and freshly ground black pepper
• 2 large carrots, peeled and cut into a medium dice (about 2 cups)
• 2 large parsnips, peeled and cut into a medium dice (about 2 cups)
• 2 medium turnips, peeled and cut into a medium dice (about 1 ½ cups)
• 1 sweet potato, peeled and cut into a medium dice (about 1 ½ cups)
• 1 teaspoon dried thyme
• ½ cup chopped onion
• ½ cup chopped celery
• 2 garlic cloves, minced
• 1½ pounds ground lamb
• 3 tablespoons flour
• ½ cup sherry, vermouth or white wine
• 1 tablespoon tomato paste
• 1½ cups chicken stock
• 1 teaspoon Worcestershire sauce
• 1 tablespoon chopped fresh rosemary leaves
• 2 tablespoons chopped fresh Italian parsley
• Salt and pepper to taste