Wash the watercress and remove the leaves. Put them to one side.
Finely chop the shallot and put it into a heavy-based pan with the oil. Soften over a moderate heat till they are translucent and the edges have just started to turn golden.
Pour in the stock and add the watercress stems. Cook for about 20 minutes till they are soft, if necessary topping up with a little water
Blitz and, if you want a 'perfect' soup, pass the mixture through a sieve.
Return to the pan and add the roughly chopped leaves. Cook for a further 2-3 minutes.
Add the Philadelphia cheese and blitz again for a few seconds. Taste and season with salt and pepper as necessary
Serve garnished with one chopped quail egg per bowlful