Valentine Warner's watercress and gruyère soufflé

Heat the oven to 220c/fan 200c/gas 7. Butter an 850ml soufflé dish and dust with 1 tsp of sifted flour – turn the dish round to coat it, then discard the excess. Cut a 10cm-wide strip of baking parchment and tie around the soufflé dish with kitchen string, leaving 5cm rising above the rim to make a paper collar.

Rinse the watercress under running water and shake off the excess. Put on a board and trim off any tough stalks. Finely chop the rest and put to one side. Melt the butter in a medium non-stick pan and stir in the flour. Cook for a few seconds then gradually add the milk, stirring constantly.

When all the milk is incorporated, stir in the watercress and cook over a gentle heat for 5-6 minutes, stirring. After this time the watercress should be very soft. Remove from the heat and add 65g of the cheese. Stir well until melted then add the egg yolks, one at a time, stirring between each addition. Season.

Whisk the egg whites in a large bowl until stiff. Fold roughly a quarter of the egg white into the cheese sauce to loosen the mixture. Tip into the bowl with the remaining egg white and fold in gently. Pour the mixture into the prepared dish and sprinkle with the reserved gruyère. Put on a baking tray and bake in the centre of the oven for 22-25 minutes, or until the soufflé is risen and golden brown. Serve immediately before it has much chance to sink. A lightly dressed green salad goes very well on the side.