Rinse and scrub the Littleneck Clams under cold water to remove grit. Place them in a large pot with about 1 cup of Bottled Clam Juice, cover, and steam over medium heat just until they open, about 5–8 minutes; discard any that remain closed. Reserve the steaming liquid, remove the clams from their shells, and coarsely chop the meat.
Slice the Salt Pork into small pieces and render it in a heavy Dutch oven over medium heat until the fat is released and the pieces are crisp; transfer the crisped pork to a paper towel, leaving the rendered fat in the pot.
Add the Unsalted Butter to the pot and melt, then add the diced Yellow Onion and chopped Celery. Cook gently until softened and translucent, about 6–8 minutes.
Sprinkle in the All-Purpose Flour and stir constantly for 2–3 minutes to cook out the raw flour taste and make a light roux.
Slowly whisk in the reserved clam steaming liquid along with about 2 cups of Whole Milk and 1 cup of Heavy Cream, smoothing out any lumps.
Add peeled and diced Russet Potatoes, a Bay Leaf, and a pinch of Thyme Leaves. Bring the chowder to a gentle simmer and cook until the potatoes are tender, about 12–15 minutes.
If the chowder seems too thick, thin it with a splash more Bottled Clam Juice or milk to reach your preferred consistency.
Stir in the chopped clam meat and any reserved clam juices, warming gently for 3–4 minutes—do not boil, or the dairy may separate.
Taste and season with Kosher Salt and freshly ground Black Pepper as needed. Fold the crisped salt pork back into the pot and remove the Bay Leaf.
Ladle the chowder into bowls, garnish with chopped Parsley, and serve with Oyster Crackers alongside for crunch.
