Melt the butter and cook the onion very gently for 10 minutes in a covered saucepan, without colouring. Stir in the flour and cook gently for 2 minutes.
Gradually add the stock, lemon juice, spinach and nutmeg. Cover, bring to the boil and simmer for about 5 minutes until the spinach has wilted.
Cool a little then puree in a liquidiser. Return to a clean pan and stir in the milk and cream. Check seasoning.
Reheat gently and serve garnished with chopped fresh herbs and a swirl of cream.