I also love this particular combination of vegetables in the filling, but you can use whatever bits you have in your fridge. Leftover roasted squash, a half-cup of beans, tiny cauliflower florets, edamame, or even something like sautéed cabbage would serve you well here. I also love Kathryn's suggestions of adding rutabaga or parsnips to the mashed potatoes!
Messing up this dish is extremely hard to do, no matter what you throw at it. If ever in doubt, sprinkle another handful of cheese to the top and call everyone to the table for dinner.
Simple Shepherd's Pie
2 pounds (about 3 large) russet potatoes
4 tablespoons butter
1/4 cup milk or cream
1 - 1 1/2 pounds ground beef, lamb, or a combination
1 yellow onion, diced
2 carrots, peeled and diced
2 celery stalks, diced
1/2 cup frozen peas
1/2 cup frozen corn
1 tablespoon tomato paste
3 teaspoons Worcestershire sauce
1 1/2 teaspoons salt, divided
1 cup shredded cheese (optional)
Heat the oven to 375°F. Set a 9x13 baking dish nearby. (You can use an 8x8 pan if you prefer thicker layers.)
Peel the potatoes and chop them into large pieces. In a medium pan, cover them with an inch or two of cold water. Place over medium-high heat and bring to a boil. Reduce the heat and simmer for 18-20 minutes, until the potatoes are completely tender.
Drain the potatoes and mash them thoroughly with a potato masher or a fork until no lumps remain. While the potatoes are still hot, mix in the butter, milk, and a half teaspoon of salt. Taste the potatoes and add additional butter, milk, and salt to taste. Set the mashed potatoes aside until ready to be used.
While the potatoes are cooking, prepare the filling. Set a skillet over medium-high heat and add just enough vegetable oil to film the bottom of the pan. When the pan is hot, add the ground beef or lamb. Break the meat apart with a spatula and stir occasionally until all the meat is crumbled and browned all the way through. Transfer the ground meat to a separate dish. Pour off all about a few teaspoons of the fat.
With the pan still over medium-high heat, cook the onions with 1/2 teaspoon of salt until they have softened and turned translucent, 6-8 minutes. Add the carrots and celery with another 1/2 teaspoon of salt, and cook until all the vegetables are tender, another 8-10 minutes. Stir in the peas, corn, tomato paste, Worcestershire sauce, and the reserved ground meat. Cook until the peas and corn are warmed through. Taste and add more Worcestershire sauce or salt as desired.
Transfer the cooked meat and vegetables into the casserole dish. Drop spoonfuls of mashed potatoes evenly over the surface, and then use a spatula to smooth the potatoes into an even layer. Create little peaks with the tip of the spatula or a fork if crispy edges are desired.
Bake, uncovered, for 30 minutes. If adding cheese, sprinkle it over the top of the casserole in the last few minutes of cooking. For an extra-crispy top, run the casserole under the broiler for 2-3 minutes before serving.
Leftovers will keep for up to a week, refrigerated.
• If you like an extra-thick layer of potatoes, either double the amount of potatoes in this recipe or cook this casserole in a smaller baking dish.