Watercress Soup from the 70s

From Good Housekeeping's Easy Entertaining.

4oz butter or margarine
1 medium onion, chopped
2 bunches watercress
2 oz plain flour
One and a quarter pints chicken stock
Half pint milk

Melt the butter in a saucepan, add the onion and cook gently for 10 mins until soft but not coloured. Wash and trim the watercress, leaving some of the stem, then chop roughly. Add to the onion, cover the pan with a lid and cook gently for a further 4 mins. Add the flour and cook gently, sitrring, for one to two mins. Remove from heat and gradually blend in the stock and milk. Bring to the boil, stirring constantly then simmer for 3 mins. Sieve or puree the soup in a food processor. Return to the pan and reheat gently without boiling. Serve hot.