Step 1In a large kettle or soup pot, cook salt pork over medium heat until fat is rendered and pork bits are crispy, about 10 minutes. Remove pork with a slotted spoon to drain on paper towels and reserve, leaving drippings in pot.
Step 2Add onion to drippings and cook, stirring frequently, until it begins to soften, about 6 minutes. Sprinkle on flour and cook, stirring, 2 minutes. Add milk and clam liquor, whisking until smooth. Add potatoes, bay leaf and dried thyme and cook uncovered until potatoes are almost tender, 10 minutes. Add clams and fresh thyme (if using) and stir in half-and-half. Continue to simmer until potatoes are tender, about 5 more minutes. Season with salt and pepper to taste. (Remove from the heat and let chowder sit, partly covered, at cool room temperature for at least 1 hour, or refrigerate for up to 2 days. Reheat gently, discarding the bay leaf.)
Step 3To serve, ladle chowder into soup bowls, add a small square of butter and stir to swirl in. Sprinkle with reserved salt pork cracklings if desired.