Preheat oven to 375°F. Spread pistachios in a single layer on a sheet pan. Toast until fragrant and lightly browned, about 5-8 minutes. Let cool completely.
Place the toasted pistachios, parmesan cheese, chopped shallot, lemon zest and juice, salt and olive oil in a food processor or blender and pulse until coarsely chopped but still quite chunky. Do not puree.
Meanwhile, bring a pot of salted water to a boil (I like to use a deep wide skillet to cook the pasta, easily toss with sauce, and then take to the table for serving). Cook the pasta according to package directions, or until al dente. Drain, reserving 1 cup of the pasta water.
Return the pasta to the pot and toss with the lemon-pistachio mixture and spinach. Add enough pasta water to make a loose sauce (I usually start with around ¼ cup and end up using about ½ cup total). Use tongs to thoroughly toss everything together and distribute the lemon pistachio mixture evenly. The spinach should wilt slightly. Add salt and pepper to taste.
Serve immediately with additional parmesan cheese, if desired.
Calories: 398kcal, Carbohydrates: 48g, Protein: 11g, Fat: 18g, Saturated Fat: 2g, Sodium: 598mg, Potassium: 389mg, Fiber: 4g, Sugar: 3g, Vitamin A: 1470IU, Vitamin C: 6mg, Calcium: 43mg, Iron: 2mg