HEAT oven to 350°F.
HEAT dressing in large skillet on medium-high heat. Add chayotes and onions; cook 15 min. or until tender, stirring occasionally. Meanwhile, heat oil in medium skillet on medium-high heat. Add tortillas, 1 at a time; cook 5 sec. on each side or just until softened. Set aside.
COMBINE enchilada sauce, corn, sour cream, 1/4 cup cheese and cilantro. Pour half into 2-qt. casserole sprayed with cooking spray; cover with layers of half each of the tortillas, chayote mixture and chiles. Repeat layers.
BAKE 20 min. Sprinkle with remaining cheese; bake 5 min. or until melted.