Cut zucchini in half lengthwise. Scoop out pulp, leaving a 3/8-in. shell. Chop pulp and set aside. In a Dutch oven, bring water and salt to a boil. Add zucchini shells; cook until crisp-tender, 5-8 minutes. Drain and set aside.
In a large skillet, saute the onion and zucchini pulp in oil until crisp-tender. Stir in the beans, corn, tomato sauce, chili sauce, cilantro and cumin. Cook over medium heat until heated through, about 5 minutes. Sprinkle with cheese; cover and cook until cheese is melted, about 1 minute. Spoon into zucchini shells; sprinkle with tomato.