Classic Lamb Shepherd’s Pie

Wash your hands and clean the preparation surfaces. Heat the oven to 400°F with a rack in the upper-middle position. Rinse and scrub the potatoes and carrots under running water, trim any damaged areas, and dry them. Rinse and dry the onion before peeling. Peel the potatoes and cut into 1-inch chunks. Peel and finely dice the carrots; finely dice the onion.

Put the potatoes in the large pot with 3 quarts water and 1 teaspoon salt. Bring to a boil, then reduce the heat and simmer for 15–20 minutes, until the potatoes are tender enough to crush easily with a fork.

Meanwhile, heat the skillet over medium-high heat. Add the lamb and cook for 5 minutes, breaking it into small crumbles and letting it develop browned patches. Add the onion and carrots and cook for another 8–10 minutes, stirring occasionally, until the onion is softened and the lamb is well browned. Spoon off excess rendered fat, leaving approximately 1 tablespoon in the skillet.

Reduce the heat to medium. Stir in the tomato paste and thyme and cook for 2 minutes. Sprinkle in the flour and cook, stirring, for 1 minute. Gradually stir in the beef broth, scraping up the browned bits from the bottom of the skillet. Add 1/2 teaspoon salt and 1/4 teaspoon pepper.

Simmer the filling uncovered for 6–8 minutes, stirring occasionally, until thick and cohesive. A spoon drawn through it should leave a trail that closes slowly, with no watery liquid pooling around the meat. Stir in the frozen peas and cook for 2 minutes. Check several places in the filling with a clean food thermometer to confirm the ground lamb has reached at least 160°F. Remove from the heat.

Drain the cooked potatoes thoroughly. Return them to the pot over low heat for 1–2 minutes, shaking gently, to evaporate excess moisture. Remove from the heat. Warm the milk and butter together in the small saucepan over low heat until the butter melts; do not boil.

Mash the potatoes, then gently work in the warm milk and butter, the remaining 1/2 teaspoon salt, and the remaining 1/4 teaspoon pepper. Stop once the mash is fluffy and smooth enough to spread; excessive mixing can make it gluey.

Spread the hot lamb filling evenly in the baking dish. Spoon the mash over the entire filling in small dollops, then gently join them together and spread to the edges. Use a fork to make shallow ridges across the potato surface to encourage browning.

Bake uncovered for 30–35 minutes, until the filling bubbles around the edges and the potato ridges are golden. Use a clean food thermometer to verify that the casserole reaches at least 165°F (74°C) in the center and several other locations. Continue baking if needed, loosely covering the top with foil if it browns too quickly. Let the shepherd’s pie rest for 10 minutes before dividing into six portions; this helps the filling settle and makes serving neater.