In a large skillet over medium-high heat, cook the bacon until crispy. Reserve half of the bacon and refrigerate it to use as a topping later.
Add the potatoes, remaining bacon, 1 tablespoon bacon grease, onion, celery, garlic, chopped clams, minced clams, clam juice, chicken broth or water, salt, black pepper, dried parsley, and dried thyme to the slow cooker. Stir everything together.
Cover and cook on low for 6–8 hours or high for 3–4 hours, until the potatoes are fork tender.
During the last 30 minutes of cooking, whisk the half-and-half and cornstarch together until smooth. Stir the mixture into the slow cooker.
Turn the slow cooker to high and cook for about 30 minutes, or until the chowder has thickened.
Ladle the chowder into bowls and top with chopped parsley and the reserved crispy bacon. Serve warm.