1Set a dutch oven over medium heat and cook the bacon, stirring now and then, for 8 to 10 minutes until crisp and the fat has rendered. Lift the bacon out with a slotted spoon onto a paper towel and leave about 2 tablespoons of the fat in the pot.
2Add the butter to the pot. When it foams, add the onion and celery with a pinch of salt and cook for 6 to 8 minutes, stirring, until soft and translucent but not browned. Stir in the garlic and cook for 30 seconds.
3Sprinkle the flour over the vegetables and stir constantly for 2 minutes to cook out the raw flour taste.
4Pour in the bottled clam juice and the reserved juice from the canned clams a little at a time, whisking after each addition so the base stays smooth.
5Add the potatoes, bay leaves and thyme. Bring to a gentle simmer, partly cover, and cook for 12 to 15 minutes, until the potatoes are tender when pierced with a fork.
6Press a few potato cubes against the side of the pot with the back of a spoon to thicken the chowder. Stir in the milk, cream and drained clams.
7Warm over medium-low heat for about 5 minutes, stirring often. Do not let it boil, or the clams turn rubbery and the dairy can curdle.
8Remove the bay leaves. Taste and season with salt and plenty of black pepper, keeping in mind the clam juice is already salty.
9Ladle into bowls and top with the reserved bacon, parsley and oyster crackers. The chowder thickens as it sits, so thin it with a splash of milk when reheating.