Heat olive oil on medium heat in a frying pan. Add diced onion and green chiles and cook until onion is translucent, about 3-4 minutes. Remove from heat and set aside.
Preheat oven to 350° F. In a large bowl, combine onion and chile mix, black beans, shredded chicken, 1 ½ cups of monterey cheese, and 1 cup of enchilada sauce. Mix well.
In a 13x9 glass pan, pour in ¼ cup of enchilada sauce and spread to cover the bottom of the pan.
Cover 3-4 corn tortillas in a damp paper towel and heat in microwave for 30 seconds. Repeat with all the corn tortillas. This will help them to not rip when assembling the enchiladas.
Assemble the enchiladas by dividing the filling between the tortillas, roll into a cylinder shape, and place in the glass pan with the tortilla seam facing down. These enchiladas should be stuffed as much as possible--there's a lot of filling!
After the enchiladas are assembled, top with the rest of the enchilada sauce. Spread over the top and sides evenly. Top with 1 ½ cups of cheese.
Bake for about 30 minutes, or until the center is warm and the cheese starts to bubble. Top with some cilantro, if you'd like. Allow to cool for 5-10 minutes before eating. Enjoy!