Step 1 Rinse the clams well under cold running water, scrubbing their shells with a brush.
Step 2 Dry the clams a bit and place them in the freezer for one hour. This kills the clams and makes them much easier to open. Shuck the clams and reserve every drop of juice that drips from them. Coarsely chop the clams and place them in a covered plastic container, separate from the clam juice. Refrigerate the clams until you are ready to finish the soup. Measure the clam juice; you will need 1 3/4 cups (make up for any shortage using canned clam broth).
Step 3Rinse the salt pork well under cold running water. Dice the salt pork into one-fourth-inch cubes. Rinse it again once it is diced, for a minimum of five minutes. Drain well and set aside.
Step 4Melt the butter in a non-reactive saucepan over medium heat. Add the salt pork to the butter and stir until the salt pork begins to render. Add the onions and cook them slowly, stirring often, until translucent (do not allow them to brown). Add the clam juice and the diced potatoes, and bring the soup to a simmer. Cook the soup just until the potatoes are done. Remove the soup from heat, taste and adjust the seasoning if needed. This makes about 1 quart of soup.
Step 5Transfer the soup to a bowl set over a bowl of ice to stop the cooking and cool the soup quickly. Cover and refrigerate until needed. The soup is best made one day ahead of time, to give the flavors time to meld.
Step 6Before serving, rewarm the soup: Bring it to a simmer, check the seasoning and add 2 ounces of chopped clams for every (1 cup) serving of chowder. Cook the clams for a minute or two while stirring. Ladle the soup into warm bowls.