01 - Place cleaned clams in a large pot with 1 cup water. Cover and steam over medium heat until clams open, about 5–7 minutes. Discard unopened clams. Remove clams, cool slightly, chop coarsely, and strain reserved cooking liquid through a fine mesh sieve.
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, stirring occasionally, until softened, about 5–7 minutes.
04 - Sprinkle flour over the vegetables and stir continuously for 2 minutes to form a roux.
05 - Gradually whisk in milk, heavy cream, clam juice, reserved clam liquid, and bay leaf. Stir in potatoes, dried thyme, salt, and pepper.
06 - Bring mixture to a gentle simmer. Cook uncovered, stirring occasionally, until potatoes are tender, about 15–20 minutes.
07 - Stir in chopped clams and half of the cooked bacon. Simmer gently for 3–4 minutes without boiling.
08 - Remove bay leaf. Adjust seasoning to taste. Garnish with remaining bacon and chopped parsley before serving.