Bring a small pan of water to the boil. Cook the green beans for 1 min, then drain and run under cold water.

Pour about 1-2 tsp oil from the tuna into a large bowl, add another 1 tbsp from a bottle, then the lemon zest and 2 tsp of the juice, along with the wine vinegar and mustard. Season and whisk until smooth.

Tip the green beans, tuna, borlotti beans, onion, celery, olives and parsley into the dressing and toss together. Serve topped with the celery leaves. If packing up for lunch, store the dressing and salad separately and toss together just before eating. Will keep chilled for a day in an airtight container.