1. Preheat broiler.
2. To prepare fish, brush fish with oil; sprinkle with 1/2 teaspoon pepper and 1/4 teaspoon salt. Place fish on a broiler pan coated with cooking spray. Broil 6 minutes or until fish flakes easily when tested with a fork or desired degree of doneness.
3. To prepare tzatziki, combine yogurt and next 5 ingredients (through garlic) in a food processor or blender; pulse until smooth.
4. Spread 2 tablespoons tzatziki in the center of each flatbread. Divide fish evenly among flatbreads. Top each serving with 2 tablespoons onion, 3 avocado slices, 2 tomato slices, and about 6 cucumber slices; fold in half.