01 - Preheat oven to 375°F. Butter a 1.5-quart soufflé dish and dust lightly with flour or grated Parmesan, tapping out excess.
02 - Melt butter in a medium saucepan over medium heat. Stir in flour and whisk constantly for 1 minute until golden.
03 - Gradually whisk in milk. Continue stirring until mixture thickens and bubbles, about 2 to 3 minutes.
04 - Remove from heat. Stir in Gruyère, Parmesan, salt, pepper, and nutmeg until cheese melts and mixture is smooth. Let cool for 2 minutes.
05 - Blend in egg yolks one at a time, then fold in chopped chives, parsley, and thyme.
06 - Using an electric mixer, beat egg whites in a clean bowl until stiff peaks form.
07 - Fold one-third of egg whites into cheese mixture to lighten. Gently fold in remaining egg whites until just combined without overmixing.
08 - Pour mixture into prepared dish. Run thumb around edge to help rising evenly.
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.