Preheat your oven to 350°F. Grease a 9 X 13-inch baking dish with the butter and set aside.
Place the chicken in a large bowl. Add 2 1/2 cups of the grated cheese, the sour cream, red onions, and some salt and pepper. Mix well.
Place the tortillas on your work surface. Spoon about 1 cup of the chicken mixture onto the center of each tortilla and roll them up. Place the rolls, seam-side down, in the prepared baking dish (it will be a tight fit). Scatter the salsa over the tortillas. Cover with aluminum foil and bake until heated through, 40 minutes.
Remove the foil and sprinkle the enchiladas with the remaining 1/2 cup of grated cheese. Return the baking dish to the oven until the cheese is melted and edges of tortilla are just beginning to get crisp, 5 to 8 minutes. Serve hot.