In a large pot, steam the littleneck clams in the water until the shells have opened. Transfer the clams to a large bowl with a slotted spoon and discard any unopened shells. Strain the juice into a bowl through a fine mesh strainer lined with a paper towel and set aside. Discard the paper towel.

In a large dutch oven, cook the bacon over medium heat until crisp and transfer to a small bowl using a slotted spoon. Drain off all but two tablespoons of fat and return to the heat.

Add the onions and celery to the pot and cook until the vegetables have softened, approximately 5 minutes. Add the garlic and cook for 1 minute. Stir in the cornstarch and cook for an additional two minutes.

Stir in the potatoes, clam juice, half and half, and milk. Bring the mixture to a boil and stir in the thyme and bay leaf. Turn the heat down to low and simmer for 15-20 minutes until the mixture has thickened a little bit and the potatoes are fork tender.

While the chowder is simmering, remove the clams from their shells and roughly chop. Add the clams and the fresh chopped clams to the chowder, stirring to combine. Continue cooking for an additional 10 minutes.

To serve, ladle into bowls and top with crumbled bacon.

To store, transfer to an airtight container and refrigerate for up to two days.