Preheat the oven to 375 degrees F. Lightly grease a 9 by 13-inch casserole dish and set aside.

In a large skillet over medium-high heat, heat 1/4 cup of the oil. Add the eggplant in batches, seasoning on both sides with salt and pepper, and cook until golden, adding more oil as needed. Transfer to a plate as they cook, and set aside.

Heat 2 tablespoons of the remaining olive oil in a large skillet over medium-high heat. When hot but not smoking, add the onion and sausage, and cook, stirring, until the sausage is browned and the onion is soft, about 6 minutes. Add the tomato paste and cook, stirring frequently, until it begins to brown, about 1 to 2 minutes. Add the tomatoes, their juices, the Italian seasoning, black olives, 1 teaspoon of salt, 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 seasoning, to taste.

Meanwhile, melt the butter in a medium saucepan over medium-high heat. Stir in the flour and cook, stirring constantly, until thickened and forms a light roux, about 2 minutes. Whisk in the milk and, whisking frequently, cook until the sauce is thickened and smooth, about 4 minutes. Remove from the heat and season with the nutmeg and salt and pepper, to taste.

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 on the white sauce, then top with the grated mozzarella and Parmesan.

Bake for 1 hour, or until puffed and golden brown on top.