This is one of those amazingly quick and simple classic Italian dishes that is easy to forget, but it was one of my favourites when i was growing up - maybe because I could make it myself as soon as my mother trusted me with the can opener and a chopping knife. It takes maybe ten minutes to throw together out of store cupboard basics, and is even better the next day if there are leftovers.
ALL YOU NEED is canned tuna (200 grams/6.7 ounces), ideally in olive oil - even more ideally, in olive oil from Italy, canned (400 gram/15 ounces) cannelloni (or flagolet) beans, red or white onions or shallots, lemons, extra virgin olive oil and parsley. Note: if what you have on hand is tuna in water or vegetable oil, that's fine, too, however you will have to adjust the olive oil to your taste.
Drain the beans (discarding the liquid) and put them in a bowl. Drain the tuna of oil (or water), saving any olive oil as you might want to add it later. Flake the tuna and add to the beans along with a chopped small-to-medium onion. Squeeze a large (or two small) lemons, adding the juice, along with salt and pepper. Finish by drizzling two or three tablespoons of olive oil (possibly using the oil from the canned tuna), stir thoroughly, and taste. Sprinkle with chopped parsley and if you have a red onion, a few thinly sliced rings on top are a nice addition.
If you are making this on a hot summer day, some beautifully ripe tomatoes sliced on the side make a perfect pairing.