In a small dish, combine yogurt with salt, pepper, oregano and garlic powder (season to taste). For some added spice, add red pepper flakes. In a skillet sprayed with no-cal cooking spray, cook onions and peppers over medium heat for 5 - 7 minutes. Add spinach and tomatoes, and cook until spinach leaves have lost their volume. Remove from heat, and mix veggies with yogurt sauce. Cut pita in half (toast if desired) and stuff with veggie mix.