Boil chicken in water until it’s thoroughly cooked. Remove from water. Let cool and then shred or cut into bite size pieces. Set chicken aside. Put onion and garlic in a skillet with the vegetable oil and sauté until softened. Add the chicken and the salsa & stir to heat through. Set aside.
Line a large casserole dish with tortillas to cover the bottom. Top with cheese, beans, chicken mixture and more cheese. Repeat again with more tortillas, cheese, beans and chicken mixture. Top with black olive slices & additional cheese.
Bake at 350°F for 35 to 45 minutes or until bubbly & browned on top. Let cool for 30 minutes & serve with sour cream.