Preheat the oven to 400°F. Coat a 9" x 13" baking dish with nonstick cooking spray.

In a medium-sized bowl, combine the chicken, 1 cup cheese, the onion, and 3/4 cup enchilada sauce; mix well. Divide the mixture evenly among the centers of the tortillas.

Tightly roll up the tortillas and place seam side down in the baking dish. Top with the remaining enchilada sauce and sprinkle with the remaining 1 cup cheese. Bake for 15 to 18 minutes, or until the cheese is melted and the enchiladas are heated through. Serve immediately.