Preheat oven to 400 degrees F.
Chop tomatillos and onion. Mince garlic. Place into a saucepan. Season with chopped cilantro, oregano, cumin, and salt; pour in water. Bring to a boil over high heat, then reduce heat to medium-low, and simmer until the tomatillos are soft, 10 to 15 minutes.
Using a blender, carefully (it will be hot so vent your blender if you need to) puree in batches until smooth.
Toss together the shredded chicken, 1/2 cup of the sauce, and 1 1/2 ounces of the Mexican cheese blend. Spoon a thin layer of the sauce on the bottom of an 8×8-inch baking dish.
Place 3 corn tortillas, overlapping as necessary, in the bottom of the baking dish. Cover with one-third of the chicken mixture; top with 2 cups of the sauce; sprinkle with 1/3 of the cotija and shredded cheese. Repeat layers, then top with remaining tortillas. Pour sauce over tortillas; sprinkle evenly with cotija and remaining shredded cheese.
Bake for 20 minutes or until thoroughly heated.
Serve with mango slices.