Poach the prawns in salted water until they are opaque in the centre and just cooked through, about two minutes. Drain and allow to cool completely.
In a large, clean pickling jar, combine all brine ingredients, fasten the jar lid and shake to combine. Add the prawns and refrigerate for two days to ensure that they’re infused with the brine flavours.
On serving day, wash the baby carrots. Cut lengthways into halves, or quarters if they’re large.
Remove prawns and onion mixture from the brine. Reserve brining mixture.
In a large bowl, combine carrots, cabbage, hazelnuts, salad leaves and brined onions with a splash of olive oil, a little of the brining mixture and some seasoning. Distribute this on the bottom of six plates and place prawns on top.