To make the dressing, combine the apple cider vinegar, mustard and honey in a small bowl. Gradually whisk in the oil and season with salt and pepper.
Cut the fennel bulbs in half lengthways and remove the core. Using a mandolin vegetable slicer, thinly slice lengthways into strips. Alternatively slice very thinly using a sharp knife. Halve and core the apple and cut into matchstick lengths, again using a mandolin, if you have one.
Place the fennel and apple in a large bowl and drizzle with lemon juice. Add the chicken, blueberries, spinach, pine nuts, dill, mint and poppy seeds. Pour the dressing over the salad and toss to coat.