Preheat your oven to 400 degrees F.
Chop the tomatillos and onion. Mince garlic.
Combine the chopped tomatillos, onion, and minced garlic in a saucepan. Add chopped cilantro, oregano, cumin, and salt; pour in water. Bring to a boil.
Reduce heat to medium-low and simmer until tomatillos are soft, about 10 to 15 minutes.
Carefully puree the mixture in a blender until smooth.
Toss together the shredded chicken, 1/2 cup of the sauce, and 1 1/2 ounces of Mexican cheese blend.
Spoon a thin layer of sauce in an 8x8 inch baking dish, then layer with corn tortillas.
Add one-third of the chicken mixture, 2 cups of sauce, and sprinkle with Cotija cheese and Mexican cheese.
Repeat layers and finish with remaining tortillas, sauce, and cheeses.
Bake for 20 minutes until heated through.
Serve warm with mango slices.
