Lentil Cottage Pie

Wash your hands with soap and water. Heat the oven to 400°F with a rack in the upper-middle position. Scrub the potatoes and carrots under running water, trim away damaged areas, and dry. Peel the potatoes and cut into 1-inch pieces. Peel and finely dice the carrots. Peel and finely dice the onion. Pick through the lentils, discard any debris, and rinse in the strainer.

Combine the lentils and 4 1/2 cups water in the saucepan. Bring to a boil, then reduce to a gentle simmer. Cook, partially covered, for 25–35 minutes, until completely tender but still holding their shape. Check periodically to ensure they are gently simmering rather than boiling dry. Drain, reserving 1/2 cup of the cooking liquid for adjusting the filling if needed.

Meanwhile, place the potatoes in the large pot with 8 cups water and 1 teaspoon salt. Bring to a boil, then reduce the heat and simmer for 15–20 minutes, until a fork passes easily through the pieces. Drain thoroughly. Return the potatoes to the pot over low heat for 1–2 minutes, stirring gently, to evaporate excess moisture.

Push the potatoes to one side of the pot. Add the milk and 3 tablespoons butter to the cleared space and warm over low heat until the butter melts. Remove from the heat. Add 1/2 teaspoon salt and 1/4 teaspoon pepper, then mash until smooth. Avoid prolonged stirring, which can make the potatoes gluey. Cover to keep warm.

While the lentils and potatoes cook, melt the remaining 2 tablespoons butter in the skillet over medium heat. Add the onion and carrots. Cook for 10–12 minutes, stirring occasionally, until the onion is soft and the carrots are nearly tender. Add the tomato paste and thyme and cook, stirring, for 2 minutes, until the paste darkens slightly.

Stir in the vegetable broth, cooked lentils, remaining 1/2 teaspoon salt, and remaining 1/4 teaspoon pepper. Simmer uncovered for 10–15 minutes, stirring frequently, until the carrots are tender and the filling is thick and moist, with no loose broth pooling around it. Mash a few spoonfuls of lentils against the skillet to help bind the mixture. If it becomes dry before the carrots soften, add the reserved lentil liquid 1 tablespoon at a time, using no more than the reserved 1/2 cup.

Spread the hot filling evenly in the baking dish. Spoon small mounds of mashed potatoes across the entire surface, then gently join them with the back of a spoon without pushing them into the filling. Spread to the edges and roughen the surface with a fork to create ridges.

Set the dish on the baking sheet to catch any drips. Bake for 25–30 minutes, until the filling bubbles around the edges and the potato ridges are golden. Check that the center reaches 165°F (74°C) with a clean food thermometer, extending the baking time if necessary to reach that temperature. Let the pie rest for 10 minutes before dividing into six portions.