Heat a large pot of water to boiling over high heat. Add rotini and butternut squash and cook for 8-10 minutes.
In a food processor or blender, chop onions and garlic. Add basil, lemon peel, salt and black pepper. Process until all ingredients are finely chopped but not pureed.
Remove 2/3 cup cooking water from the pasta/butternut squash before draining . Set aside.
Add olive oil and pasta water to the processor in a narrow stream.
Add parmesan if using, and mix into pesto.,
Drain pasta and squash. In a large serving dish, toss with pesto sauce.
Garnish with basil and additional parmesan if you like.
This recipe adapted from Good Housekeeping magazine.