01 - Boil pasta in a large pot of salted water until al dente. Reserve 1/2 cup pasta water, then drain pasta.
02 - Season shrimp with kosher salt and freshly ground black pepper.
03 - Heat olive oil in a large skillet over medium-high heat. Add shrimp in a single layer and cook 1 to 2 minutes per side until just opaque. Remove shrimp and set aside.
04 - In the same skillet, add minced garlic and optional red pepper flakes. Sauté for 30 seconds until fragrant.
05 - Pour in white wine or chicken broth, scraping up browned bits. Simmer for 2 minutes to reduce slightly.
06 - Stir in lemon zest and juice, then add spinach. Cook until wilted, about 1 minute.
07 - Return shrimp to skillet. Add drained pasta and butter. Toss to combine, adding reserved pasta water as needed to create a silky sauce.
08 - Remove from heat, sprinkle with chopped parsley, and serve immediately. Top with Parmesan if desired.