How to make shepherd's pie
- Cook the potatoes. Add the peeled and cut potatoes to a large pot of salted water. Bring to a boil, then reduce the heat slightly to prevent boiling over. Cook until fork tender, about 20–25 minutes.
- Make the mashed potatoes. Drain the potatoes well and let the steam escape for a minute or two. Mash with butter and warm milk or cream until smooth. Stir in the Parmigiano-Reggiano, then season with salt and pepper. Let cool slightly, then mix in the egg yolks.
- Brown the lamb. In a large Dutch oven over medium-high heat, add the olive oil and lamb. Cook until deeply browned, letting some crunchy browned bits form on the bottom of the pot.
- Deglaze. Add the red wine and scrape the bottom of the pot to release the fond. Once the browned bits are loosened, remove the lamb and set aside.
- Cook the vegetables. Add the onion, carrot, and celery to the same pot. Cook until softened and translucent, about 8–10 minutes. Add the garlic and cook for another minute.
- Build the filling. Stir in the tomato paste and cook for 2–3 minutes. Add the lamb back to the pot. Sprinkle in the flour and stir until everything is coated.
- Add the liquid. Stir in the Worcestershire sauce, then add enough stock to just cover the meat mixture. Bring to a boil, then reduce to a rolling simmer.
- Reduce slowly. Simmer uncovered, stirring occasionally, until the filling is very thick, about the consistency of a thick sloppy joe mixture. This can take 1½–2 hours. Do not rush it.
- Preheat the oven. Heat the oven to 425°F.
- Assemble. Transfer the lamb filling to a 9 × 13-inch baking dish. Spread the mashed potatoes over the top, sealing them to the edges. Use a fork to scrape ridges and rough texture into the potatoes.
- Bake. Place the baking dish on a sheet pan and bake until the filling is bubbling and the potatoes are golden, about 30–40 minutes. Broil briefly at the end if needed to brown the top.
- Rest before serving. Let the shepherd's pie cool for 20 minutes before slicing and serving.