Cut the chicken breasts into smaller pieces to ensure even cooking. Place them in a pot and cover with enchilada sauce. Cook on a low to medium flame until the chicken is fully cooked, about 20 minutes, stirring occasionally.
Once the chicken is cooked, remove it from the pot and shred it using two forks.
Set your oven to 375°F to get it ready for baking the casserole.
In an 8x8-inch baking dish, spread a small amount of enchilada sauce. Place two tortillas on top, cutting to fit if necessary. Add a layer of chicken, more sauce, and a sprinkle of cheese. Repeat layers twice more, ending with cheese on top.
Place the casserole in the oven and bake until the cheese is melted and bubbly, and the top is slightly browned, about 20 to 30 minutes.
Let the casserole cool slightly, then garnish with chopped green onions or cilantro if desired. Serve warm and enjoy!