Absolutely! I use half milk and half water and use finely chopped flat mushrooms. This is what I do:
Finely chop about six large, fat mushrooms. Put them into a large, heavy bottomed saucepan and saute in butter or substitute. Add a pint of chicken or vegetable stock and a paste made of water and several table spoons of cornflour. Add a pint of milk (I use semi-skimmed). Bring to the boil, then quickly lower the heat and simmer for about an hour (alternatively, pour into a slow cooker and cook as you normally would in a slow cooker). When the mushrooms are soft and the soup "tastes right", liquidise half of the mixture to get a finer and more intensively flavoured soup.
You can substitute the mushrooms for leeks and cook in the same way.
