In a large pot, cook the pasta according to the package directions, being sure to save ½ cup of pasta cooking water before draining. Do not rinse the drained pasta!

While the pasta cooks, make the pesto: Place the pine nuts in a small, dry pan set over medium-high heat and toast until golden brown, about 3 minutes, tossing them all the while to keep from burning. Place the toasted pine nuts, basil, spinach, 1 teaspoon lemon zest, 1 tablespoon lemon juice, garlic, and ½ teaspoon of salt in the bowl of a food processor and puree, as far as you can, stopping the machine and scraping down the sides at least once. Then, with the food processor running, add the oil in a slow, steady stream until the mixture takes on the right texture. Add the ¼ cup Parmesan cheese and black pepper to taste, if using, and pulse three or so times, until the mixture is just combined. Season with more salt to taste, if desired. 

Once the pesto is done, prep the ricotta: using a whisk or fork, combine the cheese with the remaining ¼ teaspoon of lemon zest and tablespoon of lemon juice. Season with salt to taste, if desired.

Return the drained pasta to the pot along with the reserved ½ cup pasta cooking water, ¾ to 1 cup of pesto (as desired), and butter. Stir until the butter melts completely and the sauce thickens. Season with salt and pepper to taste, and serve with a dollop of ricotta over the top and sprinkle of Parmesan.