Tuna and celery soufflé

Preheat to the oven to 180 ºC (350 ºF). Prepare a 1,5 litre scuffle dish.
Heat 15 ml butter in a saucepan and sauté the celery until soft. Set aside.
Heat 50 ml butter in the saucepan and add the cake flour.
Remove from the heat and stir until a smooth paste is formed. Cook for one minute, stirring continuously.
Remove from the heat and slowly add the milk, still stirring continuously.
Season with lemon juice, and salt and pepper to taste. Cool slightly in a glass bowl.
Beat the egg yolks into the mixture, one by one.
Flake the tuna and add to the mixture, along with the celery.
Beat the egg whites until stiff but not dry. Fold the egg whites into the mixture.
Turn the soufflé mixture into the prepared dish.
Run your finger round the inner edge of the soufflé dish to allow the soufflé to puff out nicely.
Bake for about 35 minutes or until the point of a knife comes out clean when inserted into the centre of the soufflé.
Serve immediately.