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1
Preheat the oven to 350u00b0.
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2
In a large pot of boiling salted water, cook the pasta until al dente, about 11 minutes (or as directed on package).
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3
Meanwhile, heat a large, heavy-bottomed saucepan over medium-high heat.
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4
Add the sausage and cook, stirring and breaking up the meat with a wooden spoon, until no longer pink, 3 to 4 minutes.
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5
Transfer the sausage to a large bowl and wipe the saucepan clean.
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6
Add the pasta and 1 cup caramelized onions to the sausage and stir to combine.
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7
In a medium saucepan or a microwave oven, heat the milk until steaming.
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8
Return the large saucepan to the stove, add the butter and melt over medium heat.
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9
Add the flour and cook, whisking constantly, for 3 minutes.
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10
Whisk in the hot milk, 1/4 cup at a time, and continue whisking until the sauce is smooth and thick and comes to a simmer.
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11
Add 2 1/2 cups cheese, remove from the heat and whisk until the cheese is melted, about 2 minutes; season to taste with salt and pepper.
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12
Add the cheese sauce to the pasta mixture and stir to combine.
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13
Butter a 9-by-13-inch baking dish and add the pasta mixture in an even layer. Sprinkle the remaining 1/2 cup caramelized onions and 1 1/2 cups cheese on top.
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14
Bake until golden and bubbling, about 40 minutes.
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15
Remove from the oven and let rest for 10 minutes before serving.
