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1
Heat a large skillet over medium-low heat.
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2
Add the pancetta and cook until cooked through and just slightly crispy, 5 to 7 minutes.
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3
Transfer to a plate lined with a paper towel and reserve.
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4
Add the onions to the skillet and saute over medium heat until soft and translucent, about 8 minutes.
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5
Set aside.
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6
Bring a large pot of salted water to a boil.
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7
Add the macaroni and cook until al dente, 6 to 8 minutes.
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8
Drain and set aside.
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9
Preheat the oven to 350 degrees F. Spray a 9-by-13-inch baking dish with nonstick cooking spray.
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10
Melt the butter in a large saucepot over medium heat.
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11
Stir in the flour and cook, stirring constantly, until lightly browned with a nutty aroma, about 5 minutes.
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12
Add the reserved onions and the garlic and cook for 2 minutes.
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13
While whisking, slow pour in the milk, making sure to whisk out any lumps.
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14
Stir in the white pepper, cayenne and 1 teaspoon salt and bring to a simmer over medium heat.
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15
Reduce the heat and let simmer, stirring occasionally, until thickened, 3 to 4 minutes.
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16
While stirring, add the grated fontina, Gruyere and Parmesan about 1/2 cup at a time and continue to stir until melted and blended.
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17
Stir in the celery leaves.
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18
Stir in the cooked macaroni.
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19
Pour the mixture into the prepared baking dish.
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20
Bake until hot, bubbly and lightly browned on top, 20 to 25 minutes, sprinkling on the cooked pancetta about 5 minutes before the macaroni and cheese is done.
