PREHEAT 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, one at a time; cook 5 sec. on each side or just until softened. Set aside.
MIX enchilada sauce, corn, sour cream, 1/4 cup of the cheese and the cilantro until well blended. Pour a little of the enchilada sauce mixture onto bottom of 2-qt. casserole dish sprayed with cooking spray. Top with 3 of the tortillas; cover with layers of half each of the chayote mixture and poblano chiles. Repeat layers.
BAKE 20 min.; sprinkle with the remaining 3/4 cup cheese. Bake an additional 5 min. or until cheese is melted.