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1
Preheat oven to 375u00b0.
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2
Butter an 8-inch square baking dish or pan or six 8-ounce ramekins; set aside.
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3
Fill a 4- to 5-quart pot about 3/4 full of water and add 1 tablespoon of the salt.
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4
Bring to a boil and add the pasta.
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5
Cook, stirring once or twice, until tender but firm, about 4 minutes, and drain; reserve the pot.
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6
While the pasta is cooking, melt 2 tablespoons butter in a medium skillet over medium heat.
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7
Turn off the heat and add the bread crumbs and Parmigiano-Reggiano; stir until mixed well; 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 about 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 similar in texture to cake batter.
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14
If it's soupy, continue cooking until it thickens.
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15
Add 1 1/2 cups of the cheddar, the Gruyere, mustard powder, cayenne, and nutmeg and stir until the cheeses have melted and the sauce is smooth but not too runny.
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16
Again, it should be similar in texture to cake batter; continue cooking if too soupy, until it thickens.
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17
Add the pasta and stir to combine.
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18
Pour into the prepared baking dish.
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19
Sprinkle with the remaining 1/2 cup of cheddar and top with the bread crumb mixture.
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20
Place the dish on a rimmed baking sheet and bake until bubbling and golden brown, about 30 minutes.
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21
Let cool for 15 minutes before serving.
