The best way to make it is with clams you have dug yourself after dropping anchor in a secluded bay, but it is awfully good with fresh clams purchased at a local marina.
Pour clams into a large pot with 1 cup water. Cover, bring to a boil, and cook until shells open, about 10 minutes. Allow to cool, then shuck clams and set aside. Pour off broth and reserve, taking care not to disturb any sediment in the bottom of the pan.
Cook salt pork in a large saucepan over medium heat until the fat runs. Add onion and cook, stirring, until tender. Pour off half the fat.
Add potatoes to the pan and stir well. Add clam broth and just enough water to cover the potatoes. Simmer until potatoes are tender.
Add milk and half-and-half; bring to a simmer. Add clams and butter; simmer a few minutes until clams are heated through and butter is melted. (Do not boil or clams will be tough.) Season with salt and pepper.