Bring a large pot of salted water to a bowl. Add the fusilli and cook until al dente according to the package directions. Drain in a colander in the sink and cool by running under cold water for a minute or so while tossing.
Once the pasta is cooled, transfer it to a large bowl with the pesto. Toss using your hands until very well coated and mixed. Finish by grating more Parmesan cheese over the pasta if you desire. If you want to sneak in some bulk and nutrition, toss with the celery root. Serve at room temperature.
Roughly chop the basil and place in a blender, pressing it down with a rubber spatula to condense. Chop the garlic scape and add to the basil, then add the olive oil and salt. Blend in 3-second pulses, stirring in between pulses, until the basil is mostly pureed. Add the butter and pulse for 5 seconds on high. Transfer to a small mixing bowl with a rubber spatula and stir in the Parmesan and pine nuts if using. Use immediately or store in an airtight container in the fridge.