For the plums:
1) Preheat griddle to high. Brush cut side of plums with oil, drizzle with 1 tbsp honey, and sprinkle with a pinch each of cinnamon and orange zest.
2) Place plums on the griddle, cut side down, and cook for 2 minutes or until golden brown and slightly caramelized. Turn over and griddle until just heated through, approximately 1 minute longer.
3) Place 3 plum halves in each of 4 bowls and top with a few tbsps of the spiced walnut yoghurt sauce.
For the spiced walnut yoghurt sauce:
1) Whisk all ingredients together in a small bowl. Cover and refrigerate if not using right away.