Cook couscous according to package directions. Transfer to a bowl and let cool. To cool the couscous more quickly, spread it out on a tray or large platter.
In a small bowl or measuring cup, whisk dressing ingredients: olive oil, balsamic vinegar, salt and pepper.
Add mozzarella, tomatoes, parsley, basil, and onion to couscous. Stir to combine. Add dressing, stirring to coat.
Best served immediately.
Pearl couscous is another name for Israeli couscous.
If desired, substitute mozzarella pearls (you won't need to halve them) or buy a log of fresh mozzarella and cut it into small pieces.
Italian parsley is also known as flat leaf parsley.
Make Ahead Tip: If you refrigerate the salad to serve later, add the basil and the dressing right before serving. If you have leftover salad, it can be refrigerated up to 3 days but the couscous will absorb some of the dressing. To refresh the salad, add more olive oil and vinegar, as needed.