01 - Slice tops off sourdough boules and hollow out centers, leaving a 1-inch thick shell. Set aside.
02 - In a large pot or Dutch oven over medium heat, cook diced bacon until crisp. Remove with slotted spoon and set aside.
03 - Add butter to the pot, then sauté onion, celery, carrots, and garlic until softened, about 5 minutes.
04 - Sprinkle flour over vegetables and cook, stirring constantly, for 2 minutes to form a roux.
05 - Gradually whisk in reserved clam juice and bottled clam juice until mixture is smooth.
06 - Add potatoes, dried thyme, bay leaf, salt, and black pepper. Bring to a simmer and cook until potatoes are tender, approximately 15 minutes.
07 - Stir in drained clams, cooked bacon, whole milk, and heavy cream. Simmer gently for 5 minutes without boiling. Remove bay leaf.
08 - Taste and adjust seasoning as needed. Stir in chopped fresh parsley.
09 - Ladle hot chowder into prepared sourdough bowls and serve immediately.