Add the basil, pepitas, and garlic to the bowl of a food processor. Process until the basil and garlic are finely chopped. Add the olive oil and lemon juice and process until smooth. Add the Parmesan and pulse just to combine. Season with salt and pepper.
Cook the pasta in well-salted water according to package directions. Drain and rinse with cold water.
Add the drained pasta to a mixing bowl with ½ cup of pesto. Toss to combine.
Add the chicken, sun-dried tomatoes, and mozzarella. Toss to combine. Garnish with fresh basil before serving.
Customize by adding halved cherry tomatoes, arugula, black olives, or bacon. Toss in extra pepitas for added crunch.
If the pasta absorbs the pesto while refrigerated, toss the salad with another spoonful of pesto or drizzle in some olive oil before serving.
Store the extra pesto in a glass container with an airtight lid. Cover the pesto with a layer of olive oil to keep the basil fresh and vibrant.