Stir the salt in 100 ml (3½ fl oz) water to make a brine. Wearing rubber gloves to protect your hands, turn the sea urchin upside down to expose the mouth. Using scissors, pierce a hole, then cut a circle, about 7-8 cm (2¾-3¼in) in diameter out of the shell. Lift up the circle and remove. Using a spoon, scoop out the white starshaped part and discard and then scoop out the orange roe. Rinse off any black stuff clinging to the roe. Place the roe in the brine and leave overnight in the fridge. Lift out of the brine to serve.