Preheat the oven to 375?F. Grease a 9 x 13-inch casserole with 1 teaspoon olive oil and set aside.
Heat 1/4 cup plus 2 tablespoons olive oil in a medium Dutch oven over medium-high heat. Add the eggplant in batches, seasoning on both sides with 1/2 teaspoon salt, 1 teaspoon Italian Essence, and 1/2 teaspoon pepper. Cook until golden brown, adding more olive oil if needed. Transfer the eggplant slices to a plate as they cook and set aside.
Heat the same Dutch oven over medium-high heat. Add the sausage and cook, stirring to break up with a wooden spoon until browned, about 5 minutes. Add the onions, tomato paste, and garlic and season with 1/2 teaspoon salt and 1/4 teaspoon black pepper. Cook, stirring frequently, until the paste begins to brown, about 5 minutes. Add the tomatoes with their juice, the remaining teaspoon Italian Essence, the black olives, and the crushed red pepper. Reduce the heat to medium-low and gently simmer, uncovered, until the flavors marry and the sauce slightly thickens, about 20 minutes. Remove from the heat. Add the basil and adjust the seasoning to taste.
Meanwhile, melt the butter in a medium saucepan over medium-high heat. Stir in the flour and cook, stirring constantly, until the mixture is thickened and forms a light roux, about 2 minutes. Whisk in the milk and cook, whisking frequently, until the sauce is thick and smooth, about 4 minutes. Remove from the heat and season with the nutmeg, 1 teaspoon salt, and the remaining 1/2 teaspoon pepper.
Combine 4 quarts water seasoned with the remaining 1 teaspoon salt and 1 tablespoon olive oil in a large heavy pot over high heat. Bring to a boil. Add the ziti and cook, stirring occasionally, until al dente, 8 to 10 minutes. Remove from the heat and drain in a colander.
Arrange half of the eggplant slices on the bottom of the casserole, overlapping as necessary. Top with half of the cooked pasta, then half of the sausage-tomato sauce. Place another layer of eggplant slices on top of the sauce, then layers of pasta and sausage-tomato sauce. Pour the white sauce evenly over the mixture, then top with the mozzarella and Parmesan cheeses.
Bake the casserole until puffed and golden brown, 30 to 35 minutes. Remove from the oven and serve hot.
Makes 8 to 10 servings