Gather the ingredients.
 

Preheat the oven to 200°C.
 

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

Stir in the flour and mustard until well combined with the leeks.
 

Gradually add the milk, stirring constantly to create a thick, glossy sauce. Once thickened, season with salt and a touch of pepper.
 

Add the egg yolks and beat well with a wooden spoon until fully incorporated.
 

Stir in the grated cheese until completely melted into the sauce.
 

In a clean bowl, beat the egg whites with an electric whisk until they form stiff peaks, which should take about 5 minutes.
 

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

Lightly grease 6 individual ramekin dishes with a small amount of butter. Gently spoon the soufflé mixture into the dishes, 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 risen.
 

Serve immediately with a green salad and crusty bread. Act quickly, as the soufflé will begin to deflate after a few minutes.

Serving Suggestions:
Individual soufflés can be served as an elegant starter, a side dish for meats and fish, or as a delightful vegetarian main course.

Raw Egg Warning:
Consuming raw or lightly cooked eggs may pose a risk of foodborne illness.