Cook the pasta: Fill a large pot with water and add a few tablespoons of salt. Bring to a boil, add pasta and cook until al dente, following the package instructions. Reserve ½ cup of pasta water and drain the rest. Return the pasta to the pot and set it aside.
Saute the garlic: Meanwhile, heat olive oil in a large skillet over medium heat. Add garlic and saute for 1 minute while stirring constantly.
Add the spinach: Add the spinach leaves, salt, and pepper to the skillet. Cook, tossing often until the spinach is wilted and has lost most of its volume.
Transfer to a food processor: Transfer the now-cooked spinach to the bowl of a food processor*. Add in the parmesan cheese, lemon juice, and the reserved pasta water.
Blend: Process until fully pureed, stopping and scraping the sides as necessary. Give it a taste and add more seasoning, if desired.
Combine with the pasta: Pour the spinach pasta sauce over the cooked pasta, and use tongs to give it a gentle toss to combine.
Serve: Serve warm with extra parmesan cheese on top and lemon zest, if preferred.
