Melt the butter in a medium-sized pan, and stir in the thinly sliced spring onions and the cubed potatoes.
Stir and cook over a gentle heat so the vegetables soften gently without browning. Now stir in the shredded lettuce. Get it all nicely coated with butter. Then add the stock, bring to simmering point, cover and cook gently for about 10 minutes or just long enough for the potatoes to soften.
Next pour the contents of the saucepan into a liquidiser. Add the chopped fresh herbs and the cream and blend until smooth. Return the puree to the pan and reheat, tasting and flavouring with the lemon juice, salt and pepper. Serve piping hot with some fresh, crusty wholewheat bread.