Begin by shredding the cabbage as finely as possible, a mandolin is best for this job but a sharp knife will do. Finely slice the red onion.  Place them into a bowl and then massage a little salt and 2 tablespoons vinegar into the shredded cabbage and red onion, add 3 tablespoons of sunflower oil, stir, let it stand for 15 mins. Prepare the other salad ingredients.

Segment the grapefruit and reserve any juice for the dressing. Cook the farro according to the packet instructions. Skin the mackerel fillets and break up into smaller pieces.

Make the dressing: add the miso to a bowl and pour any juices from the grapefruit into the bowl, add the juice of a small lemon and 3 tbls oil and a little salt, whisk. 

Arrange the salad ingredients on the plate and drizzle over the dressing.