Cook the Bacon and Vegetables:

If using bacon, render it in a large pot over medium heat until crisp. Remove the bacon and set it aside, leaving the fat in the pot.
In the same pot, add butter. Sauté the onion and celery until softened, about 5 minutes.

Make the Roux:

Stir in the flour, cooking for about 1-2 minutes until the mixture (roux) is golden but not browned.

Add Liquids and Potatoes:

Slowly pour in the clam juice or fish stock, stirring constantly to avoid lumps. Add the reserved clam juice, bay leaf, and cubed potatoes. Bring to a simmer and cook until the potatoes are tender, about 15-20 minutes.

Add Clams and Cream:

Add the chopped clams and heavy cream. Heat through, but do not boil, for about 5 minutes. Season with salt and pepper to taste.

Serve:

Discard the bay leaf. Ladle the chowder into bowls, garnish with crispy bacon (if used) and fresh parsley. Serve with oyster crackers or saltine crackers.