115g slab bacon, sliced bacon, or salt pork, chopped
2 tbsp (30g) unsalted butter
1 large onion, finely chopped
4 celery stalks with leaves, finely chopped
Kosher salt, freshly ground pepper
2 large Yukon Gold potatoes (about 500g), peeled, cut into 1/2″ pieces
250ml clam juice or fish broth
1 tbsp fresh thyme leaves
750g skinless cod or haddock fillets, cut into 2″ pieces
1L half-and-half
Cook bacon in a large Dutch oven or other heavy pot over medium heat, stirring often, until brown but not crisp, 8–10 minutes. Transfer bacon with a slotted spoon to paper towels.
Add butter, onion, and celery to drippings in pot; season with salt and pepper. Cook, stirring occasionally, until onion and celery are soft, 5–8 minutes.
Add potatoes, clam juice, thyme, and 1 cup water. Bring to a boil, reduce heat, and simmer until potatoes are tender, 10–15 minutes.
Season fish with salt and pepper and place on top of potatoes. Cover pot and cook (liquid should be barely simmering at this point) until fish is opaque throughout, 5–7 minutes (thicker pieces will take longer to cook). Add half-and-half and return to a simmer (at this point, fish will break into smaller pieces); season with salt and pepper just before serving.
DO AHEAD: Chowder can be made 2 days ahead. Let cool; cover and chill.