1. Put potatoes in a heavy, nonreactive pot with just enough water to cover. Bring to a boil, then lower heat so liquid just simmers.
2. Cook until potatoes are tender, 12 to 15 minutes, then add chopped onion and minced clams and bring to boil a second time. Boil for 1 to 2 minutes only, just enough to cook clams but not enough to make them rubbery. Remove pan from heat at once and let the mixture ripen for 20 minutes or so.
3. Return pan to low heat and add milk or cream. Cool just until chowder is hot. Do not let it boil. Taste and adjust salt. Serve in heated bowls with pats of butter on top.