Boil the pasta in salted water according to package instructions. Strain and rinse under cold water. Set aside.
Whilst the pasta is cooking prepare the pesto. Blitz together the basil leaves with the garlic, parmesan, oil and water until grainy. Season to taste and set aside.
Toast the pine nuts in a pan, stirring all the time until lightly browned. This will take less than a minute so be careful not to brown the pine nuts too much.
To assemble the salad stir the pesto through the pasta (ensure the pasta is cold), add the mozzarella balls and cherry tomatoes and stir gently. Adjust the seasoning, scatter over the pine nuts and serve.
Cook the pasta al dente, then strain and rinse in cold water to ensure it doesn't get clumpy and that you can use it straight away. If you want to cook it ahead cover and refrigerate until you are ready to use it.
You can swap orzo for another small shaped pasta variety (such as soup pasta).
If you want to make the pesto ahead add a few drops of lemon juice and keep in an airtight container in the fridge for a few hours.
I recommend adding the pine nuts into the salad (rather than pureeing them with the pesto ingredients) for a bit of crunch.
You can use sunflower seeds, crushed hazelnuts or almonds or other nuts and seeds in place of the pine nuts.
If kept in an airtight container in the fridge this salad will stay fresh for up to 2 hours.