When a piece of boneless meat or poultry is of uneven thickness, like a chicken breast or a leg of lamb, it won’t cook evenly. Thinner parts get overcooked by the time the thicker parts are done. But you can even things out by butterflying the food. To butterfly something means to cut it horizontally almost but not entirely in half, and then to open it like a book. The benefits of this cut are twofold: the meat or poultry will cook more evenly and more quickly.