01 - Bring a large pot of salted water to a boil. Cook the spaghetti according to package directions until al dente. Reserve 1/2 cup of pasta water, then drain the pasta.
02 - While the pasta cooks, pat the shrimp dry and season with salt and pepper.
03 - Heat olive oil in a large skillet over medium-high heat. Add the shrimp in a single layer and cook for 1-2 minutes per side until pink and opaque. Remove shrimp from the skillet and set aside.
04 - In the same skillet, add garlic and red pepper flakes (if using). Sauté for 30 seconds until fragrant, but not browned.
05 - Pour in the white wine (or chicken broth) and scrape up any browned bits. Simmer for 2-3 minutes to reduce slightly.
06 - Add lemon zest, lemon juice, and spinach. Toss until the spinach is wilted, about 1 minute.
07 - Return the cooked shrimp to the skillet. Add the drained pasta, butter, and Parmesan. Toss everything together, adding reserved pasta water as needed to create a silky sauce.
08 - Remove from heat. Stir in parsley and adjust seasoning with more lemon juice, salt, or pepper to taste.
09 - Serve immediately with extra Parmesan and a drizzle of olive oil, if desired.