Put the onions and vinegar in a bowl and season.
Cook the beans in boiling salted water for 3 minutes then scoop out with a slotted spoon. Cook the orzo in the same water until just tender then drain, rinse under cold water and drain really well again.
Tip the beans, orzo, olive oil, tuna, peppers, olives and dill into the bowl with the onion and season well. Toss together and serve.
