Cook bacon in Dutch oven or heavy pot over medium 5-7 minutes until crisp. Remove with slotted spoon, leaving fat.
Melt butter in fat; sauté onion and celery 5 minutes until softened, then garlic another minute until fragrant.
Sprinkle in flour; stir to coat and cook 2 minutes, stirring constantly, to remove raw taste.
Gradually whisk in stock to prevent lumps. Add potatoes and thyme; boil, reduce heat, and simmer 15 minutes until potatoes tender.
Add clams with juice, cream, and bacon, reserving some for garnish. Gently heat through 5 minutes; do not boil or cream may curdle.
Season with salt and pepper; ladle into bowls, topping with reserved bacon and parsley. Serve with crusty bread.