Fresh livers are best. If using frozen ones, let them thaw. Cut the livers in half, then they will cook more evenly. Remove all fat and white tissue. Check each liver as you cut them apart to be sure the green bile sac is removed.
Roll livers in flour, then shake off the surplus. You want just a thin film of flour.
Heat butter in a heavy skillet to hot. Add livers. Brown and toss-cook, which means to toss the food in the skillet to turn it over rather than run the risk of puncturing the food with a fork and letting the juices drain out into the skillet. When livers are browned, they are done. They are best served pink or medium, and never well done to the point of being dry. One pound is sufficient for 6 as the first course, but will serve only 3 as the main course. Season after they are cooked.