Put the chopped onion into a bowl with the lemon juice and let it steep while you get on with the salad.
Drain the beans and rinse to get rid of any gloop, then place in a bowl.
Drain the tuna and flake it into the beans.
Add the olive oil and some salt to the onion and lemon juice mixture, whisking to make a dressing, then pour it over the tuna and beans and transfer to a serving dish.
Fork the tuna and bean salad through, seasoning with salt and ground pepper, and scatter the parsley over; if you want to put this on a plate (rather than a dish) consider strewing the plate with rocket first, or make a leafy border.
