Preheat oven to 350°F. Spray a 9x13 baking dish with cooking oil spray. Set aside.
In a large bowl, combine the shredded chicken meat, 1 (10-ounce) can of the enchilada sauce, the diced green chilies, taco seasoning, salt, black pepper and 1 cup of the crumbled cotija cheese. Using a large spoon, mix to combine.
Fill and roll each tortilla by placing ½ to ¾ cup filling onto one side of the flour tortilla (you will want to create a line of chicken filling about an inch offset from the center) and start rolling from the end that has the filling.
Place the filled and rolled tortilla, seam side down, into the prepared baking dish starting at one end. You will repeat this until all the flour tortillas are filled, rolled, and placed into the baking dish.
Slowly and evenly pour the second (10-ounce) can of enchilada sauce onto the tops of the filled and rolled tortillas. Do not go all the way to the open edges of the chicken enchiladas. Leave about 1 inch of the edges of the enchiladas without sauce.
Top the sauce-covered chicken enchiladas with the cheddar and jack shredded cheese. Make sure you get an even layer.
Bake uncovered for 30 minutes or until bubbling and the cheese is melted.
Garnish your baked chicken enchiladas with a few slices of jalapeño, a sprinkle of the remaining ¼ cup cotija cheese, chopped cilantro, and a few avocado slices.
If you prefer extra saucy enchiladas, add more sauce to the mixture. For lighter flavor and texture, use less sauce, or leave it out of this step.
If you opt for yellow corn tortilla shells in your enchiladas, they need to be warmed slightly before rolling or they will break apart. Either fry them quickly in a skillet greased with a little oil, or warm up the sauce before adding it to the pan, and thoroughly coat the tortillas with sauce before filling. It is not necessary to warm flour tortillas before filling them.
Easily make these enchiladas a freezer meal by covering the pan with plastic wrap and freezing them instead of baking. Thaw overnight in the refrigerator and cook according to the instructions.