Put 1 tablespoonful of the butter into the Dutch oven with the onion over medium high heat, and saute for 5 minutes. Add the pancetta and cook for another few minutes, then remove from pan and set aside.
Add the rest of the butter to the pot, and when melted, add the flour and mix well.
Cook for about 4 minutes over medium high heat, then slowly begin adding the milk, a little at a time. If you add too much milk at first, the white sauce can turn lumpy. Continue to add milk gradually, adding a little more each time, mixing until it is completely absorbed before adding more, until it has all been added.
Next, add the water or stock, and liquid from the clams. Then stir in the pancetta, onion, and potatoes into the soup. Simmer over low heat, stirring often and scraping the bottom to make sure it doesn’t stick. Taste for salt and adjust if needed.
When the potatoes are ready, add the clams and parsley and cook for 3 minutes or so.
Taste for salt again, add some freshly ground pepper and remove from heat.
When ready to serve, cut the top off of the sourdough boules cutting deep into the bread, and remove some of the inside to form a bowl. Heat the bread in a hot oven for about 5 minutes, to make the crust crispy. Ladle hot chowder into the bread bowl. Garnish with parsley if desired and serve immediately.