Preheat the oven to 375 degrees F. Generously butter a 9 x 13-inch baking dish with 1 tablespoon of the butter.

Melt the remaining 3 tablespoons butter in a medium saucepan over medium-low heat. Add the flour and let cook, stirring constantly, without browning the flour, for about 2 minutes. Whisk in the milk 1/2 cup at a time, and season with 1/2 teaspoon of the salt, the pepper, and nutmeg. Bring to a simmer, whisking often, and cook until thick, about 5 minutes. Whisk in 1/2 cup of the grated cheese. Remove from the heat, cover, and keep warm.

Heat a large nonstick skillet over medium-high heat. Add the sausage and cook in batches until just browned, about 4 minutes. Set aside on a plate. Add the fennel to the pan and cook, stirring, until softened, about 5 minutes. Add the garlic and stir until fragrant, about 1 minute. Return the sausage to the pan and continue to cook 2 to 3 minutes longer until flavors are blended.

Meanwhile, bring 1 gallon water and the remaining 2 teaspoons salt to a boil in a large pot over high heat. Add the ziti and cook, stirring occasionally to keep the pasta from sticking together, until just al dente, about 8 minutes. Drain, then return to the pot.

Toss the pasta with the olive oil. Add the sausage mixture, the cream sauce, and basil and mix well.

Spread the pasta in the prepared dish and sprinkle with the remaining 3/4 cup cheese. Bake until golden brown and bubbly, about 30 minutes. Serve immediately.