Drain the beans, tuna and corn and place in a bowl, picking the tuna a little with a fork. Peel and squeeze the garlic and mix with the vinegar, olive oil, the chilli flakes, salt and pepper to a marinade and pour over the salad. Then let it sit for about 20 minutes. Pluck the parsley or chop it very roughly and fold it into the salad. Arrange on a platter to serve and garnish with the onion cut into rings.
It goes well with crispy, fresh baguette or ciabatta.