Bring a medium pot of salted water to a boil.
Add the orzo and cook according to the package instructions (about 8-10 minutes).
Drain and rinse under cold water to stop the cooking process.
While the orzo is cooking, halve the cherry tomatoes and dice the mozzarella if needed.
If using pine nuts, lightly toast them in a dry pan over medium heat for 2-3 minutes, until golden brown.
In a small bowl, whisk together the pesto, lemon juice, salt, and black pepper.
If the pesto is too thick, add 1 tbsp olive oil to loosen it.
In a large mixing bowl, combine the cooked orzo, cherry tomatoes, mozzarella, and arugula (if using).
Pour the pesto dressing over the salad and toss to coat everything evenly.
Sprinkle with toasted pine nuts, if using.
Serve immediately, or let it chill in the fridge for an hour to let the flavors meld.
Enjoy as a side dish or a light main course.