Place chicken in sauce pan of cold water, bring to boil, lower heat and simmer covered 8-10 minutes or until done. Remove to plate to cool slightly. Saute onion in vegetable oil (1-2 tbsp) over medium high heat until very soft and starting to brown lightly. (Save oil in pan use for softening tortillas). Using 2 forks, shred the chicken and put in large mixing bowl. Add sauteed onion to bowl. Add 1 cup shredded cheese to bowl. Add 1/2 of sliced olives to bowl and mix all ingredients thoroughly. Add salt and pepper to taste. Add 1/2 cup sour cream to bowl and mix to bind ingredients and simplify handling. Pour enough enchilada sauce in 8"X 11" baking dish to cover bottom. Fill each tortilla with chicken mixture, roll and place seam sdie down in baking dish (crowd enchiladas together closely)
Sprinkle remaining cheese over top. Distribute remaining sliced olives over top. Pour remaining green enchilada sauce over all, being sure to cover each enchilada. Bake at 350 degrees for 25-30 minutes until bubbling. Serve with sour cream.