Make an incision an eighth of an inch in depth on both sides of the fillets, lay them on a dish and let marinate in salt, pepper, lemon juice and oil. Two hours later, drain them off, roll in flour and fry firm to a fine golden color.
Ranhofer gives the method and leaves the amounts to the cook, which is normal for The Epicurean. The scoring and the drain step are what make it work.
Butterfish were a cheap, abundant catch along the American Atlantic coast in the nineteenth century, landed from New England down to the Chesapeake and sold in New York's markets. The name is usually explained by the soft, rich, oily flesh, though the derivation is traditional rather than documented; that oiliness is also a plausible reason why Charles Ranhofer, chef of Delmonico's, treats them with acid and a flour crust rather than a sauce, since the lemon cuts the fat and the frying firms flesh that poaches into mush. The Epicurean (1894) is a professional reference, and this brief entry sits among dozens of fish preparations meant to be read by someone who already knew how to run a fry station. The marinade-then-fry sequence follows French practice for marinated fish, here applied to a local American species.