01 - Preheat oven to 375°F. Butter a 1.5-quart soufflé dish and dust lightly with flour or grated Parmesan, removing excess.
02 - Melt butter in a medium saucepan over medium heat. Whisk in flour and cook for 1 minute until lightly golden.
03 - Gradually whisk in the milk and cook, stirring, until thickened and bubbling, about 2 to 3 minutes.
04 - Remove from heat and stir in Gruyère, Parmesan, salt, pepper, and nutmeg until cheese is melted and mixture is smooth. Cool 2 minutes.
05 - Stir in egg yolks one at a time, then fold in chopped chives, parsley, and thyme.
06 - Using an electric mixer, beat egg whites until stiff peaks form.
07 - Gently fold one-third of the egg whites into cheese mixture to lighten. Carefully fold in remaining whites until just combined.
08 - Pour mixture into prepared dish. Run thumb around edge to aid even rising.
09 - Bake for 25 to 30 minutes until puffed and golden brown. Avoid opening oven during baking.
10 - Serve at once to preserve height and texture.