Instructions

1. In a Dutch oven or large pot, melt 3 tablespoons of the butter over medium-high heat. Meanwhile, in a large bowl, season the lamb generously with salt and black pepper and turn to coat. When the foam subsides, add half of the lamb and cook, stirring occasionally, until browned on all sides, about 5 minutes. Using a slotted spoon, transfer to a plate; add 3 more tablespoons of the butter to the pot and repeat with the remaining lamb; set aside. To the empty pot, add the leeks and carrots and turn the heat to medium. Cook, scraping up any browned bits, until softened slightly, about 4 minutes. Add the flour, reserved lamb, and any accumulated juices and cook, stirring continuously, until no white streaks remain, about 1 minute. Add the stock, rosemary, thyme, Worcestershire, nutmeg, and salt and black pepper to taste, then turn the heat to high. When the liquid boils, turn the heat to medium-low, cover, and cook, stirring occasionally, until the lamb is tender but not falling apart, about 40 minutes. Uncover and continue to cook, stirring frequently, until the sauce has thickened and the lamb flakes when pierced with a fork, 35–40 minutes more. Stir in the peas and set aside.
2. Position a rack in the center of the oven and preheat to 375°F. To a large pot set over high heat, add the potatoes and enough salted water to cover by 1 inch. When the liquid boils, turn the heat to medium-low and cook until tender when pierced with a fork, 20–25 minutes. Drain, then return the potatoes to the pot and add 6 tablespoons of the butter, the half-and-half, and salt and black pepper to taste. Using a potato masher, mash until smooth.
3. Scrape the lamb mixture into a casserole dish and smooth the top. Top evenly with the mashed potatoes, then use a spoon to make decorative swirls. Scatter with the cubed butter and bake until golden and bubbling, about 30 minutes. Let shepherd's pie stand for 10 minutes before serving.