Preheat 400 degrees. spray 9 x 13 casserole dish with non-stick spray.
In large skillet, heat olive oil. Once hot, add the spinach, zucchini, and yellow squash. Cook, about 5 minutes, until the spinach is wilted and squash is soft. Drain off any excess liquid and place in a large mixing bowl.
Add the remaining ingredients to mixing bowl with the spinach mix. Combine well and spread the mixture in an even layer in the prepared casserole dish.
Bake for 30 to 40 minutes or until golden brown on top. Let cool slightly before serving.