Cook the pasta to al dente, according to the package instructions. Drain into a colander and rinse well under cold water. Shake out as much excess water as you can, then set on a towel to drain further while you make the pesto.
Make the pesto: Place the pine nuts and garlic in the bowl of a food processor fitted with a steel blade. Blend for 10 long pulses. Add the basil, spinach, salt, and pepper. With the processor running, slowly pour in the olive oil through the feed tube. Puree until smooth, about 45 seconds to 1 minute. Scrape down the side of the food processor bowl as needed. Blend in the Parmesan.
Shake out the pasta once more, then transfer it to a large serving bowl. Stir the Greek yogurt into the pesto (you can blend it in right in the food processor if you’re making homemade), then scoop it over the pasta. Stir gently to coat (I find it easiest to use my hands).
Add the arugula, cherry tomatoes, peas, mozzarella pearls, and Parmesan. Stir to combine (again, it's easiest to toss with your hands). If the pasta seems too thick or dry, stir in olive oil gradually as needed.
Taste the pesto pasta salad and adjust the seasoning as desired. Serve at room temperature, or press a sheet of plastic wrap against the top to deter the pesto from losing its color and refrigerate until ready to serve.
Notes
*If you use feta, the pasta salad will be saltier, so you most likely won't need to season it as much at the end.
**This recipe is a slightly reduced amount of my homemade Basil Pesto. You can also use 2/3 cup of the store-bought pesto of your choice.
***Toasting the nuts is optional, but really amps up the flavor of the pesto, so I recommend it. To toast: place the nuts in a small, dry skillet. Warm over low heat, shaking the pan often, until fragrant and toasted, about 4 minutes for pine nuts and 6 to 8 minutes for walnuts. BE PATIENT! Nuts burn when you crank up the heat.
TO STORE: Refrigerate leftovers in an airtight container for 2 to 3 days. Let it sit on the countertop for a few minutes before serving and, if needed, add a drizzle of olive oil and toss to coat to loosen it up a bit.