Heat your oven to 375ºF.
Slice the zucchini in half lengthwise. Use a small spoon to scoop out the center of each zucchini half, leaving a 1/4-inch border. Roughly chop the zucchini that you scooped form the middle. Save all but 1 cup of zucchini for another use (like chocolate zucchini muffins or even a smoothie).
Heat the oil in a large pan over medium heat. Add the onion and cool until soft, 3-5 minutes. Add the lentils, corn, spinach, and 1 cup enchilada sauce. Simmer 5 minutes, or until slightly reduced.
Pour the remaining 1/2 cup of enchilada sauce into the bottom of a 9x11 baking dish. Arrange the zucchini halves cut-side up in the dish and fill with the lentil mixture. Sprinkle with mozzarella.
Cover the baking dish tightly with foil. Bake 30-40 minutes, or until the zucchini is tender and the cheese is melted. Remove from the oven and sprinkle with cilantro.