Preheat the oven to 375°F (190°C).

Grease a souffle dish with unsalted butter, then dust with grated Parmesan cheese.

In a saucepan over medium heat, melt the butter and stir in the flour, cooking for about 1 minute to form a roux.

Slowly whisk in the milk and cook for 3–5 minutes until the mixture is smooth and thickened.

Remove the saucepan from heat and stir in the shredded Gruyère cheese, salt, black pepper, and nutmeg.

Whisk in the egg yolks until fully combined.

In a clean bowl, whip the egg whites until stiff peaks form.

Gently fold the whipped egg whites into the cheese mixture until no white streaks remain, then pour into the prepared dish and bake for 30 minutes or until golden and puffed.