Put the milk, onion, bay and peppercorns in a small pan and place over a low heat. Bring gently to the boil then take the pan off the heat and leave to infuse for 20 minutes.
Meanwhile chop the watercress until very fine – ideally, in a blender. Grease the ramekins.
Melt the butter in another small pan and add the flour. Mix well and cook for a minute or two. Strain the infused milk through a sieve into the pan. Cook the mixture, stirring, for a couple of minutes or until the sauce has thickened – if you end up with lumps, don’t panic: just pass it back through the sieve.
Off the heat, mix in the mustard, egg yolks and watercress and check the seasoning.
In a large bowl, whisk the egg whites until stiff then gently fold into the watercress mixture. Divide the soufflé mixture evenly between the ramekins.
Pop the ramekins in a strong roasting tray and add boiling water from the kettle to approximately halfway up the sides of the dishes. Bake for 15-20 minutes at Gas 4/180C or until the soufflés are set but still have a little wobble. Leave to cool, then put the dishes in the fridge for up to two days.
To reheat, turn the soufflés out onto an oven-proof tray. Sprinkle with the cheese and spoon the cream over. Bake at Gas 7/ 220C for 10 minutes or until golden brown and puffy. Serve immediately with a crisp green salad.