01 - Preheat oven to 350°F (175°C). Slice the tops from each sourdough loaf and carefully hollow out the centers, leaving a 1-inch bread wall. Arrange bowls and tops on a baking sheet and bake for 10 minutes until crisp. Set aside to cool.
02 - In a large pot over medium heat, cook chopped bacon until crisp. Transfer bacon with a slotted spoon to a plate lined with paper towels. Reserve bacon fat in the pot.
03 - Add butter to the pot with bacon fat. Sauté onion and celery for 5 minutes until softened. Add minced garlic and cook for 1 minute until fragrant.
04 - Stir in diced potatoes, then sprinkle flour over vegetables. Cook, stirring frequently, for 2 minutes to eliminate raw flour taste.
05 - Gradually whisk in bottled clam juice and reserved clam juice, scraping up any browned bits. Pour in whole milk and heavy cream. Add bay leaf and dried thyme. Bring mixture to a simmer over medium heat, stirring often.
06 - Continue to simmer, stirring occasionally, until potatoes are fork-tender, about 15 minutes.
07 - Stir in chopped clams and cooked bacon. Simmer for 3–5 minutes more. Remove and discard bay leaf. Season chowder with salt and freshly ground black pepper as desired.
08 - Ladle hot clam chowder into prepared bread bowls. Garnish with chopped parsley and, if desired, chives or extra parsley. Serve immediately with bread tops for dipping.