In a large pot or Dutch oven over medium heat, cook the bacon until crispy, about 5 minutes. Transfer bacon to a paper towel-lined plate, leaving drippings in pot.
Add butter to the pot. Once melted, add onion and celery; cook until softened, about 5 minutes. Add garlic and cook 1 minute more.
Sprinkle flour over vegetables and stir constantly for 1-2 minutes until golden.
Slowly whisk in clam juice and chicken broth, scraping up any browned bits. Bring to a simmer.
Add potatoes, bay leaves, and thyme. Simmer until potatoes are tender, about 15 minutes.
Reduce heat to low. Stir in heavy cream and clams. Cook gently until heated through, about 5 minutes. Do not boil.
Season with salt and pepper to taste. Remove bay leaves.
Serve hot, garnished with reserved bacon and fresh parsley.
Tips
For thicker chowder, mash some potatoes against the side of the pot before adding cream.
If using fresh clams, steam them open and use the liquor for extra flavor.
Store leftovers in the refrigerator for up to 3 days; reheat gently.