Best Shepherd's Pie

Directions

1. Cook the lentils in a medium saucepan of salted, boiling water until tender, 10 to 15 minutes. Drain, transfer to a bowl, and set aside.
2. Heat the oil in a deep skillet or Dutch oven over medium-high heat. Add the beef or lamb and cook, undisturbed, for 3 minutes (less stirring means more browning). Use a wooden spoon or spatula to break up the meat into small pieces, and continue to cook until browned and mostly cooked, another 5 minutes. Season with salt and pepper. Use a slotted spoon to transfer the meat to a plate and set aside.
3. There should be a decent amount of fat in the pan, but if it’s dry, add another splash of oil. Add the mushrooms and cook undisturbed for 3 minutes (again, we’re browning here!), then toss and let sit again for 2 to 3 minutes until all the mushrooms are charred. Season with salt and pepper. Stir in the onion, carrot, and fennel and sauté until they’re just starting to brown, about 5 minutes. Season with salt and pepper.
4. Stir in the thyme and tomato paste and cook, stirring frequently, until the paste darkens in color, about 90 seconds, then sprinkle the flour on top. Cook for 1 minute, then add the wine. Bring the mixture to a gentle boil, then lower to medium-low and let the wine reduce, about 5 minutes. (If you’re the skipping wine, just move onto the next step.)
5. Stir in the cooked meat and lentils, followed by the stock, peas, and Worcestershire sauce. Turn up the heat to bring the mixture to a boil, then lower to medium-low to reduce until the mixture has thickened to the consistency of ragu, 15 to 20 minutes. Transfer to a 2-quart baking dish and heat the oven to 400°F.
6. While the filling simmers, make the topping: bring a large pot of salted water to a boil (I estimate 1 tablespoon of kosher salt per quart of water). Add the potatoes and parsnips and cook until a paring knife easily pierces both vegetables, about 15 minutes. Drain into a colander then return to the pot and mash until smooth. Melt the butter in a small saucepan, then slowly whisk in the milk. Stir the butter mixture and sour cream into the mashed potatoes. Season with salt and pepper to taste.
7. Top the filling with the potato-parsnip mixture: To prevent the filling from bubbling through the topping, start from the outside of the dish to seal in the mixture. Use an offset spatula to swoop and swirl, like frosting a cake. Place the baking dish on a rimmed sheet pan and bake for about 25 minutes, or until the topping has started to brown. Turn on the broiler for 2 to 3 minutes to further brown the topping if you want. Let cool for at least 15 minutes before serving.