In a large soup pot or Dutch oven, cook the chopped bacon over medium heat until crispy, about 5–7 minutes. Remove the bacon with a slotted spoon and set it aside on a paper towel-lined plate. Leave about 1 tablespoon of bacon fat in the pot.
Add the butter to the pot with the bacon fat. Once melted, stir in the diced onion and celery. Sauté until soft and translucent, about 5 minutes. Add the minced garlic and cook for another minute, stirring constantly to prevent burning.
Sprinkle the flour over the veggies and stir well to coat. Cook the flour mixture for 2–3 minutes to eliminate the raw flour taste, stirring continuously. This step helps thicken the chowder and gives it that classic creamy texture.
Slowly pour in the clam juice and the juice from the canned clams, stirring constantly to prevent lumps. Add the diced potatoes, bay leaf, thyme, and a generous pinch of salt and pepper. Bring to a simmer and cook until the potatoes are tender, about 15–18 minutes.
Lower the heat to medium-low and stir in the milk, cream, and chopped clams. Simmer gently for another 5–7 minutes, just to warm through—do not boil, or the dairy may curdle. Adjust seasoning to taste.
Remove the bay leaf and discard. If you like a thicker chowder, gently mash some of the potatoes with a potato masher or blend a small portion using an immersion blender. Ladle into bowls, top with crispy bacon and fresh parsley, and serve with crusty bread or oyster crackers.