01 - If using fresh clams, place cleaned clams in a large pot with 1 cup water. Cover and steam over medium heat until clams open, about 5 to 7 minutes. Discard any that remain closed. Remove clams, allow to cool slightly, and chop coarsely. Reserve the cooking liquid, strain through a fine mesh sieve, and set aside.
02 - In a large Dutch oven or heavy pot, cook chopped bacon over medium heat until crisp. Transfer bacon to a paper towel-lined plate, leaving 2 tablespoons fat in the pot.
03 - Add butter to the pot, then stir in onion, celery, carrot, and garlic. Cook for 5 to 7 minutes, stirring occasionally, until vegetables soften.
04 - Sprinkle flour over the vegetables and stir continuously for 2 minutes to form a roux.
05 - Gradually whisk in whole milk, heavy cream, bottled clam juice, reserved clam cooking liquid, and bay leaf. Stir in diced potatoes, dried thyme, salt, and black pepper.
06 - Bring the mixture to a gentle simmer. Cook uncovered while stirring occasionally until potatoes are tender, approximately 15 to 20 minutes.
07 - Add chopped clams and half of the cooked bacon. Simmer gently for 3 to 4 minutes without boiling.
08 - Remove the bay leaf. Adjust seasoning to taste. Garnish with remaining bacon and chopped parsley before serving.