Begin by gathering all your ingredients.
 

Preheat your oven to 200°C.
 

In a large saucepan, gently melt the butter over medium heat. Add the finely chopped leeks and cook them slowly until they become soft but do not brown.
 

Stir in the flour and mustard until they are fully incorporated with the leeks.
 

Gradually pour in the milk, stirring continuously to form a thick, glossy sauce. Once the sauce has thickened, season it with salt and a pinch of pepper.
 

Add the egg yolks to the mixture, beating them in with a wooden spoon until they are completely blended.
 

Next, stir in the grated cheese, allowing it to melt thoroughly into the sauce.
 

In a separate, clean bowl, use an electric whisk to beat the egg whites until they reach a stiff peak, which should take about 5 minutes.
 

Using a metal dessert spoon, gently stir 2 tablespoons of the beaten egg whites into the soufflé base to lighten the mixture. Then, carefully fold in the remaining egg whites in two batches, being sure to retain as much volume as possible. Work quickly but delicately.
 

Lightly grease 6 individual ramekin dishes with a small amount of butter. Gently spoon the soufflé mixture into each dish, filling them to within 1.25 cm of the top. Place the ramekins on a baking sheet and bake in the preheated oven for 15 minutes, or until the soufflés are golden and have risen.
 

Serve immediately with a fresh green salad and crusty bread. Be quick, as the soufflés will begin to deflate shortly after coming out of the oven.

Serving Suggestions:
These individual soufflés make a delightful starter, an elegant side dish for meats and fish, or a satisfying vegetarian main course.

Raw Egg Warning:
Please note that consuming raw or lightly cooked eggs may carry the risk of foodborne illness.