01 - In a large pot over medium heat, cook the bacon until crisp. Remove with a slotted spoon and set aside, leaving the rendered fat in the pot.
02 - Add butter, then sauté onion and celery in the bacon fat until softened, about 5 minutes. Stir in garlic and cook 1 minute more.
03 - Sprinkle flour over the vegetables and stir to form a roux. Cook for 2 minutes, stirring constantly.
04 - Gradually whisk in clam juice, scraping up any browned bits. Add potatoes, bay leaf, thyme, and season with salt and pepper.
05 - Bring to a simmer, cover, and cook 15–20 minutes until potatoes are tender.
06 - Reduce heat to low. Stir in milk, cream, clams, and half of the cooked bacon. Heat gently for 5 minutes; do not boil. Adjust seasoning.
07 - Slice off the tops of sourdough bread loaves and hollow out the centers, leaving a thick shell.
08 - Ladle hot chowder into each bread bowl. Garnish with reserved bacon and fresh parsley if desired. Serve immediately.