Preheat oven to 400 degrees and spray a large baking sheet with cooking spray.
In a large bowl, stir together chickpeas and sliced red onions. Pour olive oil and all spices over the mixture, stir to toss, and spread on the prepared baking sheet. Bake 20 minutes, stirring once halfway through baking.
Make the tzatziki sauce:
Meanwhile, make the tzatziki sauce by combining all ingredients in a small bowl.
Assemble the pitas:
Assemble the pitas by filling them with the chickpea mixture, tzatziki sauce, spinach, tomato, and/or feta.