Cook clams until just opened. Strain and reserve 1 cup of broth. Finely chop clams. Fry salt pork in large Dutch oven. Remove salt pork and add onion; brown, being careful not to let it scorch. Add potatoes and enough hot water to cover. When potatoes are done, add clams, cook for 3 minutes over low heat. Slowly add milk and cream. Simmer for at about an hour being careful not to let it curdle. Add thickening made of flour and butter, season and simmer slowly for an other hour. DO NOT LET IT BOIL. Serve hot.
Notes: My grandfather even had this recipe printed in the New Bedford newspaper years ago.
If done right it doesn't curdle and is rich & creamy. He used to use cream .. but we started using a combination of milk & cream ... it's just as tasty. One other "trick" my grandmother started doing was using some instant potato flakes as a thickener in place of some of the flour. Tastes better and works as well.
Lastly ... if you like you can add the salt pork back into the chowder after adding the milk. It is very important that you keep watch over the chowder so as to prevent it from boiling as this will cause the milk/cream to curdle. If it curdles it just won't taste right. You can put in any seasonings you like ... but keep it subtle.