Preheat the oven to 200°C fan/220°C/Gas 7. Butter the inside of a soufflé dish and dust with 1 teaspoon of flour. Turn the flour around the dish and tip out the excess. Cut a strip of baking paper and tie it around the outside of the soufflé dish with kitchen string, leaving 5cm rising above the rim to make a paper collar.
Place the watercress on a board and trim off only the truly tough stalks. Pick out any flora that don’t look like watercress and remove any unwelcome fauna. Bring a saucepan of water to the boil and fill a big bowl with cold water with some ice cubes clicking about within. Drop the watercress in the pan, and when the water returns to the boil, cook it for 30 seconds, then immediately lift it out and put it straight into the icy bath. When properly cooled, drain it thoroughly and gently wring out any remaining water with your hands. Purée the watercress in a blender or in a bowl with a stick blender. Leave it to one side.
Melt the butter in a medium, non stick saucepan and stir in the flour. Cook for a few seconds, then gradually add the milk, stirring constantly with a whisk to eradicate lumps. It will all clag up initially, but persevere with the milk and all will loosen again by the time the milk is incorporated. Remove from the heat and add 65g of the grated cheese. Stir well until melted, then add the egg yolks two at a time, whisking well between each addition. Stir in the cooled watercress purée and check the seasoning one last time.
Whisk the egg whites in a large bowl until stiff. Fold roughly a quarter of the egg white into the cheese sauce to slacken the mixture. Tip the contents of the pan back into the bowl with the remaining egg white and fold in gently. Pour the mixture into the prepared dish and sprinkle with the reserved cheese. Put on an oven tray and bake in the centre of the oven for 22–25 minutes, or until the soufflé is well risen and golden brown. It should be a little runny inside. Command your guests to sit down just before it is ready. A lightly dressed green salad goes very well with it.