Brush cut side of plums with oil, drizzle with 1 tablespoon honey, and sprinkle with a pinch each of cinnamon and orange zest.
Place plums on the grill, cut side down, and grill for 2 minutes or until golden brown and slightly caramelized. Turn over and grill until just heated through, approximately 1 minute longer.
Place 3 plum halves in each of 4 bowls and top with a few tablespoons of the Spiced Walnut Yogurt Sauce.
Whisk all ingredients together in a small bowl. Cover and refrigerate if not using right away.