Heat the oil in a pan, add the onion and quinoa, cook briefly. Pour in the water, bring to the boil, season with salt, cover and simmer over a low heat for approx. 15 mins. until just soft, leave to cool.
Using a fork, mash 100 g of blackberries in a bowl. Add the balsamic, oil and mustard, stir in, season. Add the quinoa, rocket and remaining blackberries, mix.