Warm tuna and white bean salad

Method

1. Toss the onion with the lemon juice.
2. Put the beans into a medium pan, cover with the stock, then bring to a simmer. Drain, tip into a bowl, then toss through the onion and tuna.
3. Make a dressing by whisking the vinegar with the garlic, sugar and a pinch of salt and pepper. Whisk in the mustard and oil, then pour over the bean and tuna mix. Toss in the chopped parsley, if using, then serve.