Wash the clams thoroughly and remove any blemishes. They should have no odor. Heat oil in a large saucepan (with a cover) over medium heat and cook garlic and onion until translucent. Add the wine and let cook for a couple of minutes to combine the flavors. Add the clams and clam juice (or water) and cover the pan for about 10 minutes until the clams open. Discard any clams that do not open and remove clams to serving bowls. Season the juices in the pan with salt and pepper to make a sauce. Pour the sauce and melted butter over the clams.