Preheat oven to 350°.
Heat oil in a large nonstick skillet over medium-high heat. Add onion and squash; sauté 5 minutes or until tender. Add beans and next 6 ingredients; bring to a boil. Reduce heat, and simmer, uncovered, 5 minutes or until thoroughly heated.
Arrange 6 tortilla halves in bottom of an 11 x 7-inch baking dish coated with cooking spray; top with half of tomato mixture. Sprinkle with olives and cilantro. Top with remaining 6 tortilla halves and tomato mixture.
Cover and bake at 350° for 20 minutes. Uncover and sprinkle with cheese; bake an additional 10 minutes. Top each serving with sour cream.