Boil potatoes in salted water, just until fork tender. Drain, but reserve the liquid
Add clam juice to a measuring pitcher. Add enough of the reserved potato water to make 2 quarts. Add additional water if necessary to obtain 2 quarts of liquid.
Transfer clam juice/water mixture to a stock pot and bring up to 190 degrees. Remove from heat. Gradually whisk in soup base until lump free. Whisk vigorously for 1 minute. Let stand for 3 – 5 minutes and then whisk again.
While soup base is standing, cook pancetta over medium-high heat until cooked through.
Sauté onions in butter until just soft.
Add potatoes, pancetta, clams, onions, and parsley to soup base and simmer for 15 – 30 minutes, stirring frequently.
Deglaze pan used to cook pancetta with the sherry. Add liquid to soup.
Serve with oyster crackers.