1. In a large pot over medium heat, melt butter.
2. Add onion, garlic, and celery and cook until softened, about 5 minutes.
3. Add flour and whisk until lightly browned.
4. Slowly pour in the chicken broth, whisking constantly.
5. Add potatoes and bring to a boil.
6. Reduce heat to low and simmer for 15 minutes.
7. Add heavy cream, clams, thyme, and parsley and simmer for an additional 10 minutes.
8. Season with salt and pepper, to taste.
9. Serve garnished with fresh parsley.