This version of shepherd's pie calls for ground lamb (but use ground beef if you prefer) and spinach in the mashed potatoes. It's a hearty make-ahead nutritious comfort food dinner for any day of the week.
To make the filling, melt the butter in a large, deep, oven-safe skillet over medium heat. Add the onion and carrots and cook, stirring occasionally, until the onion softens and begins to brown, about 8 minutes. Add the lamb, salt, and pepper and break it up into crumbles. Cook, stirring, until the lamb is no longer pink.
Add the flour and stir until it absorbs all the moisture in the pan, about 1 minute. Stir in the stock, scraping the bottom of the pan to incorporate all of the flour. Reduce the heat to low and simmer until the carrots are tender, about 10 minutes.
Take the skillet off the heat, taste for salt and pepper, and stir in the peas. (You can prepare the shepherd’s pie filling up to 2 days in advance and reheat on the stovetop before proceeding with the recipe.)
Meanwhile, make the topping by putting the potatoes in a stockpot and add enough cool water to cover them by 1 inch. Bring the water to a boil over medium-high heat then reduce the heat to low and simmer the potatoes until they are fork tender, 15 to 20 minutes. Drain the potatoes well and return them to the pot to steam dry for 5 minutes.
lip the skins from the potatoes and pass them through a ricer or food mill or food processor, blending in the butter, and milk. Season the potatoes with the salt, pepper, and nutmeg and taste them for seasoning. Add the egg and spinach and purée or mix just until the mashed potatoes are smooth and light.
Preheat the broiler to medium high with the top rack placed 8 inches below the element. Spread and smooth the mashed potatoes over the top of the filling to make the top “crust.” Broil the shepherd’s pie until the top is evenly golden brown, 8 to 10 minutes.