Method

1. Melt the butter in a heavy based non-stick saucepan, add the flour and cook over a low heat for 3-4 minutes without colour.
2. Remove from the heat and beat in the Worcestershire sauce. Return to the heat and add the milk slowly, whisking continuously until smooth and thick.  Cook for about 8-10 minutes to remove the floury taste.
3. Allow the sauce to cool a little then beat in the egg yolks, one by one.
4. Fold in 35g / 1½ oz of the Parmesan and the cubes of Gruyère, mustard powder, nutmeg and cayenne. Season to taste.
5. Beat the egg whites until stiff and glossy. Don't over beat or the egg whites will become dry and grainy.  Stir a couple of tablespoons of the egg whites into the sauce, then fold in the remainder. Don't overwork or you will beat out the air.
6. Butter the large soufflé mould, and then sprinkle the butter with the remaining Parmesan cheese so it sticks to the edge of the dish. Spoon in the soufflé mix. 
7. With the tip of your thumb make a narrow groove around the edge of the soufflé mix, this helps the soufflé rise more evenly.
8. Place the soufflé on a flat baking tray in the centre of a hot oven 200ºC/400ºF/Gas mark 6 and cook for about 25 minutes or until golden and well risen.  Don't open the oven door to investigate!
9. The soufflé should be set around the edge and slightly soft in the centre. I like to heat double cream with a little lemon juice, salt and pepper. Make a cut in the centre of the soufflé and pour in the cream. Serve immediately.