1. Preheat oven to 350°F.
2. Lightly coat a large baking sheet with cooking spray & spread tortillas evenly over baking sheet. Lightly coat again with cooking spray.
3. Lightly coat a medium baking sheet with cooking spray. Spread fish evenly on baking sheet & sprinkle with garlic powder.
4. Bake both tortillas & fish 10-15 minutes or until fish flakes easily with a fork & tortillas are crispy.
5. Meanwhile, in a large bowl, add avocado, tomato, poblano pepper, cucumber, onion, cilantro & lime juice. Mix well. Use a fork to flake tilapia into vegetables. Mix well. Let mixture marinate 10 minutes.
6. Spoon tilapia & vegetables over tostadas, dividing evenly. Top with cheese & serve.