Pasta and Ground Lamb Casserole (Pastitsio)

Adjust an oven rack to the middle position and heat the oven to 400 degrees. Cook the lamb in a 12-inch skillet over medium-high heat, breaking the meat into small pieces with a wooden spoon, until no longer pink and the fat has rendered, about 5 minutes. Strain the lamb through a fine-mesh strainer, reserving the drippings.

Return 2 tablespoons of the reserved lamb drippings, 1 of the onions, and 1/2 teaspoon salt to the skillet and cook over medium heat until softened, 5 to 7 minutes. Stir in the tomato paste, 2 tablespoons of the garlic, oregano, cinnamon, and 1/4 teaspoon of the nutmeg and cook until fragrant, about 30 seconds. Stir in the drained lamb and crushed tomatoes. Bring to a simmer and cook over medium-low heat until the sauce has thickened, about 5 minutes. Off the heat, stir in 1/2 cup of the cheese and season with salt and pepper to taste; set aside.

Meanwhile, bring 4 quarts water to a boil in a large Dutch oven. Stir in the macaroni and 1 tablespoon salt and cook, stirring often, until just al dente, about 5 minutes. Drain the pasta, rinse under cool water until cold, and leave in the colander.

Wipe out the Dutch oven, add the butter, and return to medium heat until melted. Add the remaining onion and 1/2 teaspoon salt and cook until softened, 5 to 7 minutes. Stir in the remaining tablespoon garlic and cook until fragrant, about 30 seconds. Stir in the flour and cook, stirring constantly, for 1 minute. Gradually whisk in the milk. Bring to a simmer and cook, whisking often, until the sauce thickens and no longer tastes of flour, about 10 minutes.

Off the heat, whisk in 1/2 cup of the cheese and the remaining 1/4 teaspoon nutmeg. Season with salt and pepper to taste. Crack the egg into a small bowl. Following illustration below, whisk about 1/2 cup of the béchamel into the egg to temper, then slowly whisk the egg mixture back into sauce. Stir in the cooked macaroni until evenly coated with the sauce.

Spread half of the macaroni into a 13 by 9-inch baking dish, then spread the lamb mixture evenly over top. Spread remaining macaroni evenly over the lamb and sprinkle with the remaining 1/2 cup of the cheese. (At this point, the casserole can be covered and refrigerated for up to 2 days.)

Place the baking dish on a foil-lined baking sheet and bake until the edges are bubbling and the top is lightly golden, about 30 minutes. (If refrigerated, cover the dish with foil and bake for 30 to 40 minutes, then uncover and continue to bake for 15 to 20 minutes). Let stand for 10 minutes before serving.