A cool and peppery starter at summer's end

Being a Brit, I've always had a fondness for watercress sandwiches, the kind one has for tea with the crusts cut off, but I've always wanted to take their sophisticated, contrasting flavors from the tea table to the dinner table. Here, then, is my take on an old favorite, now transformed into an elegant cold soup for a lovely summertime lunch or dinner.

Choose your weapons
The hardware. A chopping/mashing machine of your choice—luddite that I am, I like to use a pestle and mortar, but this is not