Preheat oven to 400 degrees F. Lightly coat a 9-by-13-inch baking dish with cooking spray.
Heat oil in a large nonstick skillet over medium-high heat. Add onion and cook, stirring often, until starting to brown, about 5 minutes. Stir in zucchini, beans, tomatoes, corn, cumin and salt and cook, stirring occasionally, until the vegetables are heated through, about 3 minutes.
Scatter half the tortilla pieces in the baking dish. Top with about half the vegetable mixture, about half the enchilada sauce and half the cheese. Repeat with one more layer of tortillas, vegetables, sauce and cheese. Cover with foil.
Bake the casserole for 15 minutes. Remove the foil and continue baking until the casserole is bubbling around the edges and the cheese is melted, about 10 minutes more.