01 - Slice tops off sourdough loaves and hollow out centers, leaving about ½ inch of bread around edges. Reserve tops for serving.
02 - In a large pot over medium heat, cook diced bacon until crisp. Remove with slotted spoon and set aside, leaving 2 tbsp bacon fat in pot.
03 - Add butter to bacon fat, then sauté onion and celery for 4 to 5 minutes until translucent. Add garlic and cook for 1 more minute.
04 - Stir in flour and cook, stirring constantly, for 2 minutes to form a roux.
05 - Gradually whisk in reserved clam juice and bottled clam juice if needed to total 1½ cups liquid. Add potatoes, bay leaf, and thyme. Bring to simmer.
06 - Cover and simmer for 12 to 15 minutes until potatoes are tender.
07 - Stir in milk and cream; return to a gentle simmer without boiling. Add clams and half the cooked bacon. Simmer 5 to 7 minutes, stirring occasionally.
08 - Season with salt and pepper to taste. Remove bay leaf.
09 - Ladle chowder into bread bowls. Garnish with remaining bacon and chopped parsley if desired. Serve immediately with reserved bread tops.