In a large saucepan, add the onion, celery and potatoes. Pour the clam juice over the veggies (don't add the clams yet); add enough water just to cover the vegetables, sprinkle on salt, and simmer until tender.
MAKE THE WHITE SAUCE:
Make the white sauce when the veggies are done simmering. In another large saucepan, melt the butter; whisk in the flour. Add the two milks and whisk until thick while it's heating up. Pour the white sauce into the pan with the simmering veggies. Add the clams and season with salt and pepper, as desired. Heat through.
SERVE:
Serve the clam chowder in soup bowls and top each bowl of soup with some bacon. Serve with sourdough bread, if desired.