This is a thin, simple chowder which reminds me of my summertime favorite on Cape Cod.
Melt butter in a saucepan over medium-low heat. Cook onions in melted butter until clear, 3 to 4 minutes.
Pour clam juice into the saucepan. Add the potato; bring the mixture to a boil. Cook at a boil until potatoes are tender, about 10 minutes.
Reduce heat to medium-low. Stir heavy cream into the mixture; add clams. Continue cooking until the clams are warmed, 2 to 3 minutes. Season with salt and pepper.