Adding the parsley last, at the whizzing stage, amps up the glow
1.25 litres good quality vegetable or chicken stock
50g unsalted butter
250g trimmed watercress (leaves and fine stalks only)
300g maincrop potatoes, peeled and very thinly sliced (see Colour Code below)
sea salt and black pepper
25g trimmed flat-leaf parsley (leaves and fine stalks only)
crème fraîche or sour cream and extra leaves to serve
Bring the stock to the boil in a medium pan. Melt the butter in a large pan over a medium heat, add the watercress and stir until it wilts. Add the sliced potatoes and stir for a minute longer, then pour in the boiling stock and season with salt and pepper.
Simmer the soup for 6 minutes or until the potatoes are just tender. Whiz the contents of the pan in a blender with the parsley until smooth. Press through a sieve. Season to taste, reheat as necessary and ladle into bowls. Finish with a dollop of crème fraîche or a swirl of sour cream and a few extra leaves.
COLOUR CODE Slicing the potatoes very thinly means the soup cooks in no time and the watercress retains its greenness. Adding the parsley last, at the whizzing stage, amps up the glow.
SMART SWAPS A really useful soup: the basic method above is brilliant for any similar leafy greens. You could replace the watercress with spinach or rocket, and experiment with a mixture of two or more, flavouring with a little sorrel if available.