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Preheat oven to 400 degrees F.
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Set a large pot of water over high heat and bring to a boil.
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Set an ovenproof deep-sided skillet over medium heat. Toss in the speck and toast just until fragrant, 1 - 2 minutes. Remove speck to a plate. In the same skillet still over medium heat, melt the butter and then stir in flour. Stirring, cook until mixture edges golden, about 2 minutes. Gradually stir in milk, giving mixture time to thicken between additions. Add pepper and nutmeg. Stirring often, cook roux until very thick, 7 - 10 minutes, edging heat lower as mixture starts to steam. Remove from heat and fold in cheddar and Gruyere, stir until cheese has melted.
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Meanwhile, salt boiling water generously and cook pasta 5 minutes, add Brussels sprouts, and cook 2 minutes more. Strain pasta and Brussels sprouts, and fold into cheese sauce along with reserved toasted speck. Sprinkle top with Parmesan.
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Bake 25 - 30 minutes or until top is golden and the filling is bubbling. Cool briefly then serve.