Bring a medium-sized pot of water to a boil and add the pasta. Season with a pinch of salt and cook according to the package instructions. Drain in a colander, and spread out onto a large plate or tray. Drizzle with olive oil to prevent sticking. Place in the fridge to cool down.
Meanwhile, make the vinaigrette. In an empty jam jar, add the mustard, balsamic vinegar and olive oil. Season generously with salt and pepper. Screw the lid on tight, and give it a shake.
Slice your cherry tomatoes in half or quarter them if they're large.
Thinly slice the celery, including the leafy tops if you have any. They will add great flavor to the salad.
For the mint, stack the leaves (like a pile of paper) and roll them up (like a sleeping bag). Then, slice across into ribbons. Do the same with the basil leaves.
Grab a large bowl and add your cooled orzo, tomatoes, celery, mint, basil and mozzarella balls.
Drizzle with some of the vinaigrette and toss well. Taste your salad, and adjust with extra seasoning or vinaigrette, if necessary.