Broccoli and Watercress Soup

More soup, but this time a lovely vibrant green one for spring. Watercress is supposed to cleanse the blood, and what with the antioxidants and vitamins in the other main ingredients - potatoes and calabrese broccoli - this seems like a good way to detox and fight a bit of a spring cold I have picked up.  Of course, sploshing a couple of tablespoons of cream in probably undid it all, but sod it!  Usual start: large chopped onion or leek, sweated in about 25g of butter (butter has anti-viral properties, so perfectly acceptable!).  Add a good-sized head of calabrese broccoli, stalk and all, chopped.  Now add a couple of medium-sized, peeled potatoes, chopped.  I like a good floury one, like King Edward's.  Stir into the buttery juices for a few minutes then pour in about 750mls of veg stock - Marigold is fine - and quite a bit of black pepper, which echoes the watercress (wait for it!).  Simmer for 20 minutes, then throw in a bunch or bag of watercress, stalks too.  Simmer for another minute or two, then blend with a stick blender until all combined.  Stir in a little cream or milk and serve quickly to preserve the vivid green.