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1
Heat the oven to 400 degrees F and arrange a rack in the middle.
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2
Bring a large heavy-bottomed pot or Dutch oven of heavily salted water to a boil over high heat.
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3
Meanwhile, place the tortilla chips in a food processor fitted with a blade attachment and process into fine crumbs (you should have about 1 cup); set aside.
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4
(Alternatively, place the chips in a resealable plastic bag, press out the air, and seal the bag.
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5
Using a rolling pin, crush the chips into uniform fine crumbs.)
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6
Add the pasta to the boiling water and cook according to the package directions or until just al dente.
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7
Drain in a colander and rinse with cold water, making sure to stir the pasta while rinsing, until cooled and the shells are no longer sticking to each other; set aside in the colander.
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8
Dry the pot (no need to rinse), add 1 tablespoon of the butter, and place over medium-high heat until melted.
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9
Add the chorizo and cook, breaking up the meat into small pieces with a wooden spoon, until cooked through, about 5 minutes.
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10
Remove with a slotted spoon to a medium bowl; set aside.
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11
Reduce the heat to medium and add the remaining 4 tablespoons of butter.
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12
When the butter is melted, add the onion and cook, stirring occasionally, until softened, about 6 minutes.
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13
Add the flour, measured salt, mustard, and cayenne and cook, stirring occasionally, until the flour has darkened slightly in color, about 2 minutes.
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14
While whisking constantly, slowly add the milk to the flour mixture until evenly combined and smooth.
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15
(It will get very thick when you first add the milk, then thin out.)
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16
Cook, stirring occasionally, until the mixture comes to a simmer and thickens slightly, about 8 to 10 minutes.
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17
Add the cheeses and stir until completely melted and the sauce is smooth.
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18
Stir in the reserved pasta and chorizo and the jalapenos or green chiles and cook, stirring occasionally, until the pasta is heated through and steaming, about 4 to 6 minutes.
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19
Taste and season with salt as needed.
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20
Transfer the mixture to a 13-by-9-inch baking dish and spread into an even layer.
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21
Sprinkle with the crushed chips and bake until bubbling around the edges, about 20 minutes.
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22
Remove to a wire rack and let cool for 5 minutes before serving.
