Melt butter in a large sauce pan over medium heat. Saute onion till they start to sweat. Add evaporated milk, cans of clams in juice and the jars of clam juice. Place over medium low heat.
In three separate sauce pans boil your celery, potato, and your carrots in water till they are about 3/4's way done. I have found that if you cook them all the way from start to finish in the soup the flavor of these three things will out weigh the clam flavor of your soup.
Once the vegies have cooked drain water and add to soup base. Cook over low heat till viggies are fully tender. Add potato buds to thicken and cook for another 15-20 minutes and enjoy.