Melt the butter and cook the onion in it till soft but not brown. Stir in the potatoes and stock (I use a veg. stock for this but he recommends chicken). Add seasoning. Simmer until the potatoes are cooked. Meanwhile cutaway any think stems from the sorrel and wash it well.
Then basically all you do is add the sorrel leaves once everything is cooked - off the heat - and stick it through the blender. Because you don't need to cook the sorrel it comes out a great bright green colour. You can reheat it gently but don't boil or you loose that colour, then add the cream and chives etc. as garnish and there you go!
I found this recipe in the great Christopher Lloyd's book GARDENER COOK, it's handy for when there's not much else in the garden and sorrel is still going strong. It has a nice refreshing lemon taste.