Preheat oven to 190 degrees Celsius (375 degrees Fahrenheit).
In a large bowl, combine the shredded chicken, half of the enchilada sauce, chopped onion, and cilantro (if using). Mix well.
Warm the tortillas slightly to make them pliable. You can do this in a dry skillet or in the microwave.
Spoon about 0.25 cup of the chicken mixture into the center of each tortilla. Roll up tightly and place seam-side down in a greased 9x13 inch baking dish.
Pour the remaining enchilada sauce over the rolled tortillas. Top with the shredded cheese.
Bake for 20-25 minutes, or until the cheese is melted and bubbly and the enchiladas are heated through.
Let stand for a few minutes before serving. Garnish with extra cilantro and a dollop of sour cream or guacamole, if desired.
Notes
Serve hot with your favorite toppings like sour cream, avocado, or a side salad.