USES: Steamed or boiled, most typically as a stew vegetable, though occasionally cooked and used in salads. FINDING: Most well-stocked groceries; nearly all Mexican groceries. CHOOSING: Chayotes with unblemished skin that shows no sign of shriveling are ones of highest quality. Some Mexican cooks prefer the dark-green spiny chayotes, saying their flesh is sweeter and less watery. STORING: In the refrigerator, loosely wrapped, for a week or more.