1.Boil, steam or microwave potatoes and beans, separately, until tender; drain, cool.
2.In a medium bowl, meanwhile, make dressing by combining yoghurt, rind and juice.
3.Quarter potatoes; add to dressing with tuna, onion, beans and parsley, stir to combine. Serve salad topped with egg.
The salad can be made the night before; store, covered, in the fridge. To make the perfect hard-boiled eggs, place eggs in a small saucepan and cover with cold water. Bring to the boil; boil, uncovered, for 3½ minutes, drain. Shell eggs when cool enough to handle. Prick the potatoes all over with a fork if you’re cooking them in a microwave oven, otherwise they’ll explode.