01 - Preheat oven to 350°F (175°C). Slice the tops off the sourdough loaves and hollow out the centers, leaving about ½-inch thick walls. Place bowls and tops on a baking sheet and toast in the oven for 10 minutes until slightly crisp. Set aside.
02 - In a large heavy pot over medium heat, cook the diced bacon until crispy, about 5 minutes. Remove bacon with a slotted spoon and set aside. Discard all but 2 tablespoons of the bacon fat.
03 - Add butter to the pot. Sauté onion and celery in the fat until softened, 5 minutes. Stir in garlic and cook for 1 minute.
04 - Sprinkle flour over the vegetables and cook, stirring, for 2 minutes to form a roux.
05 - Gradually whisk in reserved clam juice, bottled clam juice, and bay leaf. Add potatoes and thyme. Bring to a simmer, cover, and cook for 12–15 minutes, until potatoes are tender.
06 - Reduce heat to low. Stir in the clams, cooked bacon, milk, and cream. Simmer gently for 5 minutes (do not boil), until heated through. Season with salt and pepper to taste. Remove bay leaf.
07 - Ladle chowder into the prepared sourdough bowls. Garnish with parsley and extra black pepper if desired. Serve immediately.