Make the slaw a bit early and let it sit at room temperature while you prepare the rest of your meal; the slaw tastes better the longer the flavors are allowed to develop.
Combine the cabbages in a large bowl.
In a small bowl, whisk together the sugar, yogurt, and lemon juice. Spoon over the cabbage and toss to combine thoroughly. Sprinkle the poppy seeds into the bowl.
Season with salt and pepper and serve.