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1
Fill a 4- to 5-quart pot about 3/4 full of water and add 1 tablespoon of the salt.
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2
Bring to a boil and add the pasta.
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3
Cook, stirring once or twice, until tender but firm, about 4-6 minutes, and drain; reserve the pot.
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4
Melt 2 tablespoons butter in a medium skillet over medium heat.
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5
Add the bread crumbs and Parmigiano-Reggiano and cook, stirring constantly, until the crumbs are a deep golden brown, 8-10 minutes.
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6
Watch carefully because they can burn easily; the crumbs will continue to crisp as they cool.
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7
Remove immediately from the heat and set aside.
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8
Using the same pot you used to cook the pasta, melt the remaining 3 tablespoons butter over medium heat.
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9
Add the onion and cook, stirring occasionally, until soft and translucent, about 5 minutes.
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10
Slowly whisk in the flour and stir constantly until the onion is coated with the flour, 30-45 seconds.
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11
Continue stirring for 1-2 minutes more, or until the mixture starts to darken slightly and smell a bit nutty.
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12
Slowly whisk in the milk, cream, and remaining 1 tsp salt and cook until the mixture is just beginning to thicken and bubble around the edges, 5-7 minutes.
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13
It should be thick enough to coat the back of a wooden spoon.
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14
Add the Gruyere, cheddar, mustard powder, cayenne, and nutmeg and cook until the cheeses have melted and the sauce is smooth but not too runny.
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15
It should be similar in texture to cake batter; continue cooking if too soupy, until it thickens.
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16
Add the pasta and stir to combine.
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17
Ladle the mixture into individual bowls and sprinkle with the toasted bread crumbs; serve right away.
