01 - Cook diced bacon in a large heavy-bottomed pot over medium heat until crisp, approximately 5-7 minutes. Remove bacon using a slotted spoon and reserve. Retain rendered fat in pot.
02 - Add butter to bacon fat. Sauté chopped onion, celery, carrot, and minced garlic until vegetables soften, about 5 minutes, stirring occasionally.
03 - Sprinkle all-purpose flour over vegetables and stir continuously for 2 minutes, creating a paste-like roux that will thicken the chowder.
04 - Gradually whisk in reserved clam juice, bottled clam juice, whole milk, and heavy cream. Add diced potatoes, dried thyme, bay leaf, and seasoning with salt and pepper.
05 - Bring mixture to a gentle simmer. Cover and cook for 15-20 minutes, stirring occasionally, until potatoes are completely tender when pierced with a fork.
06 - Remove bay leaf from pot. Stir in reserved clams and cooked bacon. Simmer gently for 5 minutes without boiling. Adjust seasoning with salt and pepper as needed.
07 - Slice off tops from sourdough loaves and hollow out centers, leaving 1/2-inch thick shells. Warm bread bowls in a 350°F oven for 5 minutes if desired for enhanced texture.
08 - Ladle hot clam chowder into warm sourdough bread bowls. Serve immediately, garnishing with reserved bacon pieces or fresh chopped parsley if desired.