Slice up the fish into small pieces and place into a small bowl. Squeeze the lemon juice over the fish trying to cover much of it as possible. Season with salt and pepper and leave to marinade and cool in the fridge.
Grate the carrot and radishes together. Finely slice the spring onion and add to the carrot and leave to one side.
Finely chop the parsley and put that in a separate bowl.