2) Remove the prawns from the pan, reserving the garlic and oil. Chop the prawns into pieces slightly larger than the beans. Toss the prawns with the beans, adding the reserved oil and garlic.
3) In a serving bowl, toss the prawns and beans with the remaining olive oil and the red onions. Strip the rosemary leaves from the stems and add to the beans. Toss, and then add the vinegar and season with salt and pepper. Toss again, before adding the basil.
4) Just before serving, toss the beans once more and adjust seasoning. Add more oil, salt, pepper, and balsamic vinegar, as desired.