Thaw salmon, if frozen. For marinade, whisk together 1/2 cup orange juice, 1/4 cup lime juice, 1/2 teaspoon chili powder, and the pepper in a small bowl.

Rinse the salmon; pat dry with paper towels. Place the salmon in a resealable plastic bag set in a shallow dish. Pour the marinade over the salmon. Seal the bag; turn to coat the salmon. Marinate in the refrigerator for 30 minutes.

Meanwhile, preheat oven to 400 degrees F. For avocado dressing, halve, seed, and peel the avocado. Combine half of the avocado, the scallions, the remaining 2 tablespoons orange juice, the remaining 1 teaspoon lime juice, the honey, oil, and salt in a food processor. Cover and process until nearly smooth. Combine the avocado dressing and kale in a large bowl, tossing until the kale is well coated.

Drain the salmon, reserving the marinade. Bring the marinade to boiling in a large nonstick skillet over medium-high heat. Add the salmon, skin side up; reduce the heat. Simmer, covered, for 8 to 12 minutes or until the salmon flakes easily when tested with a fork. Remove from the heat. Remove and discard the skin from the salmon. Transfer the salmon to a medium bowl and break into bite-size pieces. Spoon some of the warm marinade over the salmon; reserve the remaining marinade.

Meanwhile, lightly coat both sides of tortillas with cooking spray. Place the tortillas directly on oven rack. Bake for 4 to 5 minutes or until the tortillas are crisp and starting to brown, turning once.

To serve, divide the tortillas among four serving plates. Top with the kale mixture and the salmon. Chop the remaining avocado half and sprinkle over tostadas. Drizzle with the remaining warm marinade. Serve immediately. If desired, sprinkle with additional chili powder and serve with lime wedges.