Quarter the chayote and remove and discard the center seeds. Cut into 1/4-inch slices. Bring a large pot of salted water to a boil, add the chayote and boil for 5 minutes. Drain.
Heat the olive oil in a large skillet over moderate heat. Cook the onions until translucent, about 5 minutes. Then add the garlic and cook 1 to 2 minutes longer. Add the chayote, tomatoes and water. Bring to a simmer, cover and cook over low heat 10 minutes. Serve immediately.
c.1997, M.S. Milliken & S. Feniger, all rights reserved