In a small skillet, toast the pine nuts, shaking the skillet occasionally, over moderately low heat until golden, about 5 minutes. Set aside 1 tablespoon and place the remainder in a large bowl.
Get a large pot of salted water heating to a boil on the stove.
In a food processor, pulse 2 cups of the spinach until finely chopped; scrape into the large bowl with the pine nuts. To the same bowl, add the olive oil, garlic, red pepper flakes and Parmigiano-Reggiano. Season with salt and pepper and stir to combine.
Cook the pappardelle according to package directions. Once cooked and drained, stir the pasta into the pesto mixture. Add the zucchini and toss carefully to coat everything evenly.
Stir in the lemon juice and remaining 2 cups of spinach.
Transfer the pasta mixture to a serving platter. Sprinkle on the remaining pine nuts and then add a few shavings of Parmigiano-Reggiano. Serve.