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1
Preheat the oven to 350 degrees F. Move a rack to the middle of the oven.
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2
Coat a 9-by-13-inch baking dish with nonstick cooking spray.
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3
In a large pot set over high heat bring 1 gallon of water with 2 tablespoons kosher salt to a boil.
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4
Add the penne and cook 2 minutes less than the package directions, stirring occasionally.
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5
The pasta will be slightly underdone.
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6
Use a measuring cup or ladle to remove 1/3 cup pasta water and set aside.
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7
Drain the pasta into a colander.
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8
Return the pasta to the pot and set aside.
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9
Melt 4 tablespoons of the butter in a large skillet set over medium heat.
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10
Add the flour and cook for 3 to 4 minutes, whisking constantly.
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11
Pour in the milk, whisking constantly until the mixture starts to thicken like heavy cream and then becomes almost pudding-like, 2 to 3 minutes.
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12
Add the shredded cheese and whisk until smooth, the sauce will be very thick.
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13
Add the nutmeg and season with some salt and pepper.
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14
Combine the asparagus, pasta, reserved 1/3 cup pasta water and cheese sauce and toss well.
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15
Season with salt and pepper, then transfer to the prepared baking dish and top with a layer of the cheese slices.
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16
Cut the remaining 2 tablespoons butter into 1-inch cubes.
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17
Sprinkle the panko over the cheese slices, and top with the butter cubes.
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18
Bake until the top is golden brown and crisp, 20 to 25 minutes.
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19
Remove from the oven and cool for 10 minutes before serving.
