Melt the margarine in a large saucepan and fry the onions and garlic for 3-4 minutes until softened. Add the mushrooms to the pan and fry for a further 5 minutes until tender. Stir in the flour and cook for 1-2 minutes. Gradually add the stock, stirring continuously. Stir in 1 tablespoon of tarragon. Bring slowly to the boil, then reduce the heat and simmer for 10-15 minutes. Allow the mixture to cool slightly, then puree in a food processor or blender. Rub through a sieve. Just before serving add the milk or cream, season and reheat, without boiling. Pour into bowls and sprinkle over the remaining tarragon. NOTES : A delicious creamy soup which is dairy free.