Kettle of fish dating website
a fire is kindled, and live salmon thrown into boiling kettles”.
The latter is much older, dating from the eighteenth century, while yours is nineteenth century and seems to be derived from it.
Nobody is really sure where the expression comes from, but we do know that the phrase was originally a literal term.
These days, especially in Britain and Commonwealth countries, we think of a kettle as a small enclosed container with a handle and spout for boiling water to make our tea.
(I believe that Americans are less familiar with this essential item of kitchenware.) In the eighteenth century, though, a kettle was any large vessel used to boil stuff in.
There was, it seems, a custom by which the gentry on the Scottish border with England would hold a picnic (though that term was not then known) by a river.