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Preheat the oven to 375°F with the rack in the center.
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To prepare the pan: Using a pastry brush, spread the 1 tablespoon softened butter on the bottom and up the sides of a 7 1/2" (1 1/2 quart) ramekin. Sprinkle the buttered ramekin with the 2 tablespoons Parmesan cheese and shake to fully coat the sides. Set the ramekin aside.
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To make the soufflé batter: In a medium saucepan, melt the butter over medium heat. Add the flour, salt, pepper, mustard, and nutmeg; whisk to combine. Cook the mixture while stirring, for 1 to 2 minutes, or until small bubbles begin to appear on the surface. Slowly add the milk while continuing to whisk. Cook, while still whisking, for another 2 to 5 minutes or until the mixture thickens and begins to bubble. Once bubbling, cook for 1 more minute, whisking all the while. Remove the pan from the heat and whisk in the Gruyère, Parmesan, and herbs until the cheeses are melted and the mixture is well combined. Whisk in the egg yolks until fully combined (reserve the whites for the next step). Set the mixture aside to cool slightly; it will be thick.
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In the bowl of a stand mixer fitted with the whisk attachment or working in a large mixing bowl with an electric hand mixer, whip the egg whites and cream of tartar or lemon juice until stiff peaks form, 3 to 5 minutes.
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Using a whisk, mix 1/3 of the beaten egg whites into the cheese mixture until no white streaks remain. Fold this mixture into the remaining beaten egg whites. A whisk, rather than a spatula, will incorporate the egg whites in fewer strokes.
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Pour the soufflé batter into the prepared ramekin (it should not reach the top). With a clean thumb or towel, wipe away any butter, Parmesan cheese, or soufflé batter from the exposed edge of the ramekin to prevent burning.
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Bake the cheese soufflé for 35 to 40 minutes, or until it has risen 2" above the rim of the ramekin. For best results, resist the urge to open the oven door while the soufflé bakes.
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Remove the cheese soufflé from the oven and serve immediately. (The soufflé will start to sink about 2 minutes after being removed from the oven, so don’t wait!)
