INGREDIENTS
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unsalted butter, divided
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panko (Japanese breadcrumbs)
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finely grated Parmesan
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fresh thyme leaves
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kosher salt, divided, plus more
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cavatappi or other short curly pasta
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whole milk
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small onion, grated
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garlic clove, finely grated
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all-purpose flour
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grated Fontina cheese
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grated Gruyère
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grated sharp white cheddar
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English mustard powder (such as Colmanâ??s)
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Pinch of cayenne pepper