1 - Peel the onions, cut them in half from top to bottom and finely slice into half-moon pieces.

2 - Put in a colander placed over a bowl and sprinkle with salt, lightly turning over the onion pieces with your hands so the surfaces are all covered.

3 - Set aside for an hour or so to brine.

4 - Meanwhile put the vinegar, 50ml/2fl oz water and the sugar in a saucepan.

5 - Bring to a simmer, stirring to help the sugar dissolve, and cook for a couple of minutes.