Preheat oven to broil with rack 5 to 6 inches from heat. Heat oil in a large nonstick skillet over medium-high. Add zucchini, squash and onion, and cook, stirring often, until vegetables are tender and just beginning to brown, about 10 minutes. Add garlic, and cook 1 more minute. Add chicken, salt, pepper and ¾ cup of the cheese; stir to combine. Cook until hot and cheese melts, about 1 minute. Remove from heat, and cover to keep warm.
Warm tortillas according to package directions. Place about ⅓ cup of chicken mixture in the center of each tortilla; fold tortilla around filling, and place, seam-side down, in a lightly greased (with cooking spray) 11 x 7-inch (or a 2-quart) broiler-safe baking dish. Pour salsa over enchiladas, and sprinkle with remaining ¼ cup cheese. Broil in preheated oven until hot and bubbly, about 1 ½ minutes. Garnish with cilantro.
