Put the onion in a small bowl, cover with water, sprinkle with the salt and leave to soak for 20 minutes or so. Rinse well, drain and pat dry with kitchen paper before putting back in the bowl.
To make lemon fillets, slice the tops and bottoms off the lemons. Sit the lemons on a board and, with a small sharp knife, cut downwards to remove the skin and pith. Holding the lemon over a bowl