Ingredients

• 675g / 1½lb leeks, washed, trimmed and sliced into thin rings
• 50g / 2oz butter
• juice of 1 lemon, juiced
• 6 eggs, separated
• 25g / 1oz Gruyère, grated
•   mustard
•   seasoning

Method

1. Wash and trim the leeks. Be sure to get rid of the soil by making slashes where you see grit under the layers and rinsing under running water. Slice into thin rings.
2. Melt the butter in a heavy-based frying pan (which has a handle that won't melt under a grill) and fry the leeks until soft. Add the lemon juice and season well.
3. Remove the pan from the heat, add the cheese and stir until melted. Beat together the egg yolks, then beat them into the sauce along with the mustard and plenty of seasoning. Set aside.
4. Whisk the egg whites until stiff but not dry, then mix a spoonful into the cheese sauce. Carefully fold in the remaining egg whites using a metal spoon and pour into the prepared soufflé dish.
5. Bake for 25-30 minutes until well risen and golden brown. Check the soufflé by opening the oven door very slightly and nudging the dish — if the soufflé is very wobbly, cook for a few more minutes.

Notes