Preheat the oven to 375?F. Generously butter a 9 x 13-inch baking dish with 1 tablespoon of the butter.
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Melt the remaining 3 tablespoons butter in a medium saucepan over medium-low heat. Add the flour and let foam, 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.
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Heat a large nonstick skillet over medium-high heat. Add the sausage and cook until just browned, about 4 minutes. Add the fennel and cook, stirring, until wilted, about 3 minutes. Add the garlic and stir until fragrant, about 1 minute.
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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.
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Toss the pasta with the olive oil. Add the sausage mixture, the cream sauce, and basil and mix well.
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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.
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Yield: 6 to 8 servings