1. For the steak marinade, combine the curry leaves, pepper, cumin, chile powder, ginger puree, garlic puree, canola oil, and soy sauce in a blender and blend until smooth. Put the flank steak in a shallow dish and rub all over with the marinade. (A lage zip lock bag works well too, and can be thrown out after marination.) Cover and refrigerate for at least 4 hours, but no more than 7 hours.
2. For the raita (Indian yogurt sauce), combine the yogurt with the ginger and horseradish. Fold them in with a rubber spatula and season to taste with salt, pepper and sugar. Cover and refrigerate until ready to plate.
3. Grill the steak, turning once, to the desired temperature. Transfer to a platter and let rest until cooled. Thinly slice against the grain.
4. While the steak cools, brush the flatbread with olive oil and grill until warm and slightly crispy. Cut the bread into small triangles for individual servings.
5. To assemble, place a small slice of the flank steak artistically on each bread triangle. Spoon the raita sauce on top of the steak and sprinkle with chives as a garnish.