Core and thinly slice the apple. Drain the juice from the mandarin oranges into a cup, reserving 3 tablespoons of juice for the dressing.
Combine all the salad ingredients in a large bowl (see notes below) and mix. Dress with mandarin-poppy seed dressing right before serving.
Mix all the ingredients for the dressing in a small jar or food storage container. Shake well before use. Store leftover dressing in the refrigerator for 2-3 days.