Preheat your oven to 400°F. Brush a 1 1/2-quart soufflé dish with 1 tablespoon of melted butter.

In a medium saucepan over medium heat, melt the remaining 3 tablespoons of butter. Stir in the flour and whisk for about 2 minutes until it forms a smooth paste. Do not let it brown.

Gradually whisk in the milk until the sauce thickens and is smooth. Remove the pan from heat and stir in the Parmesan and Dijon mustard along with salt and pepper.

Let the sauce cool slightly. Then, whisk in the egg yolks one at a time until well combined. Fold in the grated Gruyère cheese.

Whisk the egg whites with a pinch of salt in a clean bowl until stiff peaks form. Then, gently fold them into the sauce in three additions, being careful not to deflate the batter.

Remove the soufflé from the oven and serve right away while it is still airy and light. It will deflate quickly after baking. Garnish with chives or parsley.