01 - Bring a large pot of salted water to a boil. Cook spaghetti according to package instructions until al dente. Reserve 1/2 cup of pasta water, then drain.
02 - Pat shrimp dry and season with salt and black pepper.
03 - Heat olive oil in a large skillet over medium-high heat. Add shrimp in a single layer and cook 1-2 minutes per side until pink and opaque. Remove and set aside.
04 - In the same skillet, add garlic and red pepper flakes if using. Sauté for 30 seconds until fragrant, avoiding browning.
05 - Pour in white wine or chicken broth. Scrape skillet to loosen browned bits. Simmer 2-3 minutes to reduce slightly.
06 - Add lemon zest, lemon juice, and baby spinach to the skillet. Toss until spinach wilts, about 1 minute.
07 - Return shrimp to skillet. Add drained pasta, butter, and Parmesan cheese. Toss, adding reserved pasta water as needed to achieve a silky sauce.
08 - Remove from heat. Stir in chopped parsley. Adjust seasoning with additional lemon juice, salt, and pepper to taste.
09 - Serve immediately, garnished with extra Parmesan and a drizzle of olive oil if desired.