Bring a large pot of salted water to a boil.
Add pasta and cook according to package instructions until al dente.
After pasta has been cooking for about 2 minutes, melt the butter, in a large skillet, over medium-high heat.
Add the pistachios, garlic and capers; cook, stirring to keep the garlic from scorching, until fragrant; about 1 minute.
Add spinach to skillet one handful at a time; season with salt; cook, tossing, until wilted; about 1 or 2 minutes.
Using a slotted spoon, transfer pasta to the skillet along with 1/2 cup pasta water.
Season with salt and pepper; cook over medium-high, tossing, until liquid reduces and coats the pasta; 2 to 3 minutes; season with salt and pepper.
Serve topped with Parmesan.
