Heat oil in a 5- to 6-qt. pot over medium heat. Cook onion, celery, carrot, and garlic until softened, about 5 minutes. Transfer vegetables to a plate.
Cook bacon in pan, stirring, until crisp. Transfer bacon to paper towels to drain; discard all but 1 tbsp. drippings.
Add flour and cook, whisking constantly, until slightly browned, about 1 minute. Slowly add milk, whisking constantly and letting mixture bubble between additions. Add potatoes, clams, juice, salt, pepper, and reserved vegetables.
Simmer until potatoes are tender when pierced, about 25 minutes. Add thyme and half-and-half. Sprinkle with bacon.
Note: Nutritional analysis is per serving.