Heat oil in a large nonstick skillet over medium heat. Add prechopped onion; cook 3 minutes or until tender, stirring frequently. Stir in cumin, chili powder, garlic salt, and pepper; cook 1 minute. Stir in beans; cook 2 minutes. Remove from heat. Add chicken, 1/2 cup green onions, and 7 tablespoons ripe olives; stir well.
Preheat oven to 350°.
Layer 6 tortillas on bottom of a 13 x 9-inch baking dish coated with cooking spray. Spread 2 cups chicken mixture over tortillas; sprinkle with 3/4 cup cheese. Repeat layers once. Top with remaining 6 tortillas.
Combine milk, chopped cilantro, and salsa in a blender, and process until smooth. Pour over top of tortillas. Cover and bake at 350° for 35 minutes. Uncover, sprinkle with remaining 1/2 cup cheese, remaining 1/2 cup green onions, and remaining 1 tablespoon olives. Bake for 5 minutes or until cheese melts. Serve with sour cream. Garnish with cilantro sprigs, if desired.