Gather together all of the ingredients and spray a 9x13-inch baking pan with cooking spray.
Melt the butter in a small pan and add 1 teaspoon of the garlic and the Mexican oregano and cook for 1 minute. Remove from heat and set aside.
Cut the rolls in half lengthwise and place the bottom half in the pan.
Preheat the oven to 350° F.
Heat the olive oil in a frying pan and cook the onions and remaining 2 teaspoons of the garlic over low heat until translucent, or brown them for more flavor.
Add the chicken to the onions and stir in the enchilada sauce. Simmer for 2 minutes.
Spread the enchilada chicken mixture over the rolls edge to edge and top with the cheese.
Place the tops of the rolls on top the chicken and cheese.
Stir the butter mixture and brush over the tops of the rolls. Use all of it. Tent a piece of foil over the rolls and place in the oven. Bake for 15 minutes covered, then remove the foil and bake for an additional 3 to 5 minutes until the tops have slightly browned.
Remove from oven and sprinkle with the cilantro. Let cool just a few minutes before topping with avocado slices (optional). Use a spatula to serve the sliders.