Instructions

1. Arrange a rack in the middle of the oven and heat to 400°F.
2. Place 3 pounds peeled and diced russet potatoes in a large (3- to 4-quart) pot. Cover with about 2 quarts of water, stir in 1 teaspoon of the kosher salt, and bring to a boil over high heat. Reduce the heat to medium-low, cover, and simmer until the potatoes are tender, 15 to 18 minutes. Meanwhile, make the filling.
3. Heat the oil in a large oven-safe skillet — 12-inch cast iron is best — over medium-high heat until shimmering. Add 1 1/2 pounds lean ground beef and kosher salt, breaking it into large chunks as you add it to the pan. Let the meat brown for 1 to 2 minutes undisturbed, then use a spatula to break up the chunks into smaller pieces. Cook, stirring occasionally, until browned, about 10 minutes total.
4. Add 1 (10-ounce) bag of frozen vegetables, 1/2 cup water, 2 tablespoons all-purpose flour, 2 tablespoons ketchup, 1 teaspoon onion powder, and 1/2 teaspoon garlic power. Cook, stirring occasionally, until simmering, 2 to 3 minutes. Remove from the heat. Don’t worry too much if the browned beef mixture looks a little soupy -- it will thicken in the oven.
5. Drain the potatoes and return them to the now-empty pot. Add 1/2 cup milk, 4 tablespoons unsalted butter, and remaining 1 teaspoon kosher salt and mash to desired consistency (for shepherd’s pie we like a pretty smooth mixture). Stir in 1 egg yolk.
6. Use a large spoon to dollop the mashed potatoes over the beef. Use the spoon to smooth the potatoes into an even layer.
7. Bake until the potatoes just begin to brown and the filling is bubbling, 25 to 30 minutes. Cool for 10 minutes to let the sauce thicken slightly before serving.