Prepare the trout as for a branzino (see “Dressing a Roundfish”) by scaling and gutting. Remove the head. Cut off the fins but leave the tail. Rinse well under cold running water.
The technique illustrated here is used when making Pesce alla Griglia.
To butterfly, set the fish on its backbone and slide the knife along one side of the rib cage, then along the other side, to release the rib cage from the meat. Grasp the fish at the tail end, and cut through the backbone without cutting through the skin and meat of the bottom fillet.