01 - In a large heavy-bottomed pot over medium heat, cook bacon until crisp. Remove with a slotted spoon and set aside, leaving rendered fat in pot.
02 - Add butter to pot. Sauté onion, celery, carrot, and garlic in bacon fat and butter until vegetables are softened, approximately 5 minutes.
03 - Sprinkle flour over vegetables and stir continuously for 2 minutes to form roux base.
04 - Gradually whisk in reserved clam juice, bottled clam juice, milk, and heavy cream. Add diced potatoes, thyme, bay leaf, salt, and pepper.
05 - Bring to simmer, cover, and cook for 15-20 minutes, stirring occasionally, until potatoes are tender.
06 - Remove bay leaf. Stir in clams and cooked bacon. Simmer gently for 5 minutes without boiling. Adjust seasoning to taste.
07 - Slice off bread tops and hollow out centers, leaving 1/2-inch shell. Warm bowls in 350°F oven for 5 minutes if desired.
08 - Ladle hot chowder into bread bowls. Serve immediately, garnished with bacon or chopped parsley if desired.