How to Make It
Bring clams and 4 cups water to a boil in a large pot over high. Cook until clams open, 8 to 10 minutes. (Discard any clams that do not open.) Using a large slotted spoon, transfer clams to a large baking sheet lined with paper towels; set cooking liquid aside. Let clams stand until cool enough to handle. Pull meat from shells; discard shells. Coarsely chop clam meat and set aside.
Heat oil in a Dutch oven over medium. Add onion, celery, and garlic; cook, stirring often, until onion is translucent, about 8 minutes. Add reserved clam cooking liquid, potatoes, miso, thyme, pepper, and bay leaf; cook until potatoes are tender, about 25 minutes.
Transfer 2 cups of the chowder to a food processor; pulse until coarsely chopped, about 6 times. Stir mixture into remaining chowder.
Whisk together cornstarch and remaining 1 tablespoon water in a small bowl until smooth. Stir cornstarch mixture into chowder; bring to a boil over medium-high. Remove from heat; discard bay leaf. Stir in clam meat and half-and-half until combined. Divide chowder evenly among 6 bowls. Top with chives.