4 tablespoons unsalted butter, at room temperature
2 large celery ribs, sliced crosswise 1/4 inch thick
1 large onion, coarsely chopped
1 green bell pepper, coarsely chopped
2 cups clam broth
1 quart water
2 bay leaves
2 thyme sprigs
3 dozen cherrystone clams, scrubbed
1 1/4 pounds red-skinned potatoes, scrubbed and cut into 1-inch cubes
1/4 cup all-purpose flour
3 tablespoons chopped parsley
Freshly ground black pepper
In a large skillet, cook the bacon over moderate heat until golden brown, about 4 minutes. Add 2 tablespoons of the butter and when it melts, add the celery, onion and green pepper and cook over moderately high heat, stirring occasionally, until softened, about 4 minutes. Add the clam broth and bring to a boil. Transfer the mixture to a slow cooker. Add the water, bay leaves, thyme sprigs and clams. Cover and cook on high for 4 hours, stirring a few times.
Transfer the clams to a large bowl. Discard the bay leaves and thyme sprigs and any clams that haven't opened. Add the potatoes to the broth. Cover and cook on high for 45 minutes, until the potatoes are tender.
Meanwhile, remove the clams from their shells and coarsely chop the meat.
In a small heatproof bowl, blend the remaining 2 tablespoons of butter with the flour to make a paste, then gradually whisk in 1 cup of the hot broth. Whisk the mixture into the chowder. Cover and cook, whisking a few times, until the chowder has thickened, about 15 minutes. Add the clams to the chowder, stir in the parsley and season with black pepper. Ladle into bowls and serve.
The chowder can be refrigerated for up to 3 days. Reheat gently.