Rinse the quinoa in a sieve under running water until the water runs clear, then cook for 20 mins following pack instructions. Leave to rest for 5 mins. Rinse again and drain well by pressing in the sieve.
Meanwhile, put the vinegar and oil in a large bowl, add the cavolo nero and massage in the dressing with your hands to soften it.
Add the quinoa, onion, pepper and raisins, then toss well. Spoon half onto plates, scatter over 1 chopped avocado, crumble in half the feta and toss. The remainder will keep chilled for up to three days. Top with the remaining avocado and feta, then serve.
