Cut each chicken breast into about 3 pieces so it cooks faster.
Place the chicken pieces in a small pot and pour all of the enchilada sauce over them; cover and simmer over low–medium heat, stirring occasionally, until the chicken is cooked through, about 20 minutes.
Remove the cooked chicken and shred it with two forks; reserve the sauce left in the pot.
Preheat the oven to 375°F (190°C).
Spoon about 1/4 cup of the reserved enchilada sauce over the bottom of an 8×8-inch baking dish.
Place 2 corn tortillas on the bottom of the dish, cutting them to fit if needed.
Top the tortillas with one third of the shredded chicken, one third of the remaining sauce, and one third of the shredded cheese.
Repeat the layers two more times (2 tortillas, chicken, sauce, cheese) to create three total layers, ending with cheese on top.
Bake the casserole uncovered in the preheated oven for 20 to 30 minutes, until bubbly and the cheese is melted and lightly browned.
Let the casserole rest briefly, then garnish with chopped green onions or cilantro if desired and serve warm.