Step 1Scrub clams well under cold running water. Place in a large pot along with garlic and water. Steam clams just until opened, about 6 to 10 minutes, depending on their size. Drain and shell clams, reserving broth (there should be about 4 cups).
Step 2Mince clam flesh and set aside. Strain clam broth through cheesecloth and set aside.
Step 3In a large soup pot, fry salt pork over medium heat until crisp. Remove cracklings and set aside. Add onions and saute in fat over medium heat, stirring frequently, until cooked through but not browned, about 6 minutes. Stir in flour and cook, stirring, 3 minutes. Add reserved clam broth and fish stock and whisk to remove any flour lumps.
Step 4Bring liquid to boil, add potatoes, then reduce heat and simmer until potatoes are tender, about 15 minutes. Stir in reserved clams, salt pork cracklings and cream. Heat chowder until hot. Serve in large soup bowls with oyster crackers, if desired.