Clean the clams and discard any which do not close when tapped.
Heat a large lidded frying pan until hot, add the clams and the water and bring to the boil. Cover with a lid and cook for 2-3 minutes, or until all the clams have opened - discard any which remain closed.
Strain the clams, reserving the cooking liquour. Remove all the clams from their shells and set aside, discarding the shells.
Heat the butter in another deep frying pan, add the bacon and fry for 2-3 minutes, or until browned.
Add the onions and cook for another 2-3 minutes until just softened.
Add the potatoes along with the milk, cream, bay leaf, nutmeg and cooking liquor from the clams. Bring to the boil, then reduce the heat and simmer gently until the potatoes are cooked but still just firm.
Stir in the clams and season, to taste, with salt and freshly ground white pepper.
To serve, divide the chowder among four serving bowls, sprinkle over the parsley and serve with sourdough bread.