Tropea Onion Salad

Tropea onions from Calabria have a very mild flavour and are usually eaten raw, cut into chunks and sprinkled with olive oil and vinegar. I also use them to make this salad. They are hard to find outside Italy but you could use any sweet, mild onions for this dish. Serve it on its own or, if you like, enrich it with canned tuna and hard-boiled eggs.