Gather the ingredients.

Heat the oven to 395 F / 200 C.

Gently melt the butter in a large saucepan. Add the finely chopped leeks and cook gently until the leeks are soft but not brown.

Stir in the flour and mustard into the leeks.

The, add the milk a little at a time, stirring thoroughly to create a thick, glossy sauce. Once the sauce is thickened, season with salt and a little pepper.

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

Finally add the grated cheese and stir until all the cheese is melted.

In a scrupliously clean bowl beat the egg whites with an electric whisk until stiff and firmly set (this may take about 5 minutes).

Using a metal dessert spoon, stir 2 tablespoons of the egg white into the soufflé base to loosen the mix. Then, carefully fold in remaining egg whites in 2 parts. Be very gentle and try to retain as much volume as possible. You will need to work quickly but delicately.

Lightly grease 6 individual ramekin dishes with a little butter. Gently spoon the mixture into the dishes filling to within ½ inch of the top. Place the ramekins onto a baking sheet and cook in the preheated oven for 15 minutes or until the soufflés are golden and risen.

Serve immediately with a green salad and crusty bread. Be quick though, the soufflé will sink in a few minutes. Individual soufflés can be served as starter dishes, or, as a side dish for meats and fish. The soufflé also makes a great vegetarian dish.