In a large pot or Dutch oven over medium heat, cook the bacon until crispy, about 5 minutes. Remove with a slotted spoon and set aside, leaving the rendered fat in the pot.
Add the onion and celery to the pot and cook until softened, about 5 minutes.
Sprinkle the flour over the vegetables and stir constantly for 1 minute to cook out the raw flour taste.
Gradually pour in the clam juice and heavy cream, stirring constantly to prevent lumps.
Add the potatoes, bay leaf, and thyme. Bring to a simmer, then reduce heat to low and cook until potatoes are tender, about 15 minutes.
Stir in the clams and cook for 2 minutes until heated through. Do not boil after adding clams or they will become tough.
Remove the bay leaf. Season with salt and pepper to taste.
Ladle into bowls, garnish with reserved bacon and parsley, and serve hot.
Tips
For a thicker chowder, mash some of the potatoes against the side of the pot before adding clams.
If using fresh clams, steam 2 pounds of littleneck clams in 1 cup water until opened, then chop and use the steaming liquid as part of the clam juice.
Store leftovers in the refrigerator for up to 3 days. Reheat gently to avoid curdling the cream.