Heat the oil in a 10-inch skillet over medium-high heat. Add the zucchini and onion and cook for 5 minutes or until they're tender. Stir in the spinach and corn. Cook and stir until the spinach is wilted.
Spread 1/2 cup enchilada sauce in a 3-quart shallow baking dish. Line the bottom of the baking dish with 6 tortillas, overlapping as needed. Spread with half the remaining enchilada sauce. Top with the vegetable mixture and half the cheese. Top with the remaining tortillas and enchilada sauce.
Bake for 20 minutes. Top with the remaining cheese and bake for 5 minutes or until the cheese is melted. Sprinkle with the cilantro before serving, if desired.