Cook the bacon in a Dutch oven over medium heat until crisp. Transfer the bacon to a plate and keep the rendered fat in the pot.
Add the onion and celery and cook until softened.
Stir in the butter, then sprinkle in the flour and cook briefly to make a light roux.
Whisk in the clam juice and reserved clam liquid until smooth.
Add the potatoes, bay leaf, and thyme. Simmer until the potatoes are tender.
Lightly mash a few potato pieces in the pot to naturally thicken the chowder.
Reduce the heat to low. Stir in the clams, milk, cream, salt, black pepper, and white pepper.
Warm gently without boiling. Stir in dry sherry, if using.
Let the chowder rest briefly, then serve with the reserved bacon and fresh parsley.
