1/4 pound salt pork, pancetta, or bacon, small dice (1/4 inch, about 3/4 cup)
2 celery stalks, very fine dice (1/8 inch about 3/4 cup)
8 celery leaves (use the tender yellow leaves from the core of the celery)
1 medium Yukon potato, peeled and diced (1 inch, about 1/2-2 cups)
2 leeks, thinly sliced into half moons (1/8 inch)
1 cup corn kernels, fresh or frozen
18 littleneck clams, rinsed and scrubbed
2 cups whole milk
1 cup heavy whipping cream
1 lemon squeezed (about 1/4 cup)
1 bay leaf (optional)
2 sprigs thyme (optional)
1/2 cup water plus 2 Tablespoons
2 Tablespoons finely chopped parsley leaves
Oyster crackers (optional)
Keep cod at room temperature on a plate lined with paper towels while preparing the chowder.
Rinse, peel, and dice the potatoes. Place potatoes in a 2-quart pot and cover with the milk and cream. Add the bay leaf and thyme sprigs (these will be removed when the potatoes are cooked). Bring to a simmer over medium-low heat. Cook the potatoes until tender. You should be able to easily pierce the potatoes with the tip of a knife.
While the potatoes are cooking, place the clams in a bowl and cover with cold water. Scrub the clams with a brush under cold, running water to make sure they are free of debris.
Place clams in a shallow 16-inch pot. Turn the heat on high and pour 1/2 cup of water over the clams. Cover and steam until all the clams have opened. Discard any clams that didn't open.
Let clams cool; once the clams are cool remove them from their shells and strain the cooking liquid through a fine, mesh strainer. Save the cooking liquid.
Dice the salt pork, pancetta, or bacon and add to a shallow 16-inch pot. Add remaining 2 tablespoons of water and cook over medium-low heat until all the fat has rendered.
While the salt pork is cooking, remove the top green ends of the leeks and discard. Slice the leeks from the top down, leaving the last inch of the root end intact, and rinse the leeks under cold water, cleaning each layer.
Trim the root end off the leeks and thinly slice them into half moons. Trim both ends of celery stalks, rinse under cold water, and dice.
Add the leeks and celery to the salt pork and sweat until tender. Add the potatoes, milk, cream, the cooking liquid from the clams and leeks, and bring to a simmer. Add half of the lemon juice to the chowder and season, to taste, with salt and pepper.
Season the cod on both sides with salt and pepper.
Add the corn to the chowder and stir.
Place the cod on top of the chowder and cover with a lid. Cook the cod at a light simmer for 3 minutes. Remove lid and stir in the clams; place lid back on for about 2-4 more minutes, until cooked is cooked and clams are warmed. Cooked cod should appear opaque and flaky.
Sprinkle parsley onto the chowder and stir; garnish the chowder with celery leaves and the remaining lemon juice. Serve the chowder hot with oyster crackers on the side.