Preheat oven to 350 degrees F (175 degrees C). Lightly grease a 9x13 inch baking dish.
Step 2
In a medium bowl, mix the chicken, onion, 1 cup Cheddar cheese, and 3/4 cup sour cream. Disperse the mixture evenly among the 8 tortillas. Roll into enchiladas, and arrange in single layer in the prepared baking dish.
Step 3
In a saucepan over low heat, melt together the remaining Cheddar cheese and sour cream. Pour over the enchiladas, and top with tomatoes and olives.
Step 4
Bake in the preheated oven for 20 to 30 minutes, or until hot and bubbly. Garnish with tomatoes and olives.