Preheat oven to 200 C / Gas 6. Line a baking tray with foil; lightly coat with oil or cooking spray.
Stir together yoghurt, spring onions, lime juice, 1/2 teaspoon salt and 1/4 teaspoon pepper in a large bowl. Stir in coleslaw mix. Cover and refrigerate until chilled, about 1 hour.
Arrange sweet potatoes on prepared tray. Drizzle with 1 tablespoon oil. Sprinkle with chilli powder and 1/4 teaspoon salt; toss to coat. Bake in preheated oven until tender and browned, 20 to 25 minutes.
Stir together egg and water in a shallow dish. Mix crushed cornflakes, remaining 1/4 teaspoon salt and remaining 1/4 teaspoon pepper in another shallow dish. Dip each fish fillet in egg mixture, letting excess drip off; dip in cornflake mixture, turning to coat.
Heat remaining 1 tablespoon oil in an oven-safe pan over medium-high heat. Add fish and cook until just golden, about 3 minutes. Turn fish. Transfer pan to oven. Bake until fish flakes easily, about 15 minutes. Serve with sweet potatoes and coleslaw.