Cook the orzo according to packet instructions, drain and refresh under cold water. Set aside.
Slice the onion very thinly and place it at the bottom of the bowl. add the vinegar and olive oil. All this to soak for 5 minutes. Add the pasta and the pesto and mix well. Seas on to taste.
Add all the other ingredients, toss and season again to taste.
Notes
Make this salad in advance to allow the flavours to develop.