Cook the pasta. Bring a large pot of salted water to a rolling boil. Add the pasta and cook until just al dente, following package time minus about 1 minute. Reserve 1/4 cup of the starchy pasta water, then drain well.
Dress while warm. Transfer the hot pasta to a large bowl.
Add the pesto, olive oil, lemon zest, and 1 tablespoon lemon juice. Toss gently. If it seems tight, splash in a bit of the reserved pasta water until glossy and well coated.
Season to taste. Add a pinch of salt, freshly ground black pepper, and red pepper flakes if using.
Taste and adjust with more lemon or pesto as you like.
Fold in the add-ins. Let the pasta cool for 5–10 minutes so the cheese won’t melt completely. Fold in the cherry tomatoes, mozzarella pearls, and half of the Parmesan shavings.
Add greens and nuts. Gently mix in the arugula and toasted pine nuts (if using). The arugula should soften slightly but keep some bite.
Finish and rest. Top with the remaining Parmesan shavings.
Let the salad rest for 10–15 minutes to let the flavors mingle. Serve at room temperature or slightly chilled.