To 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 when the veggies are done simmering. In another large saucepan, melt the butter, and then add the flour, mixing well with a wire whisk. Add the milks and whisk until slightly thickened. Add the clams, simmered vegetables and more salt, as desired. Heat through.
The soup will thicken more upon standing. Serve in soup bowls with a chunk of crusty sourdough bread.
Notes
As an alternative to using canned clams, if you have access to fresh clams, use 1 pound of shelled clams + 1 bottle of clam juice.
We like to sprinkle red wine vinegar on top of our clam chowder. Try it!