Bring a large pot of lightly salted water to a boil.
Add the fettuccine and cook 8–10 minutes until al dente, then drain and set aside.
Meanwhile, heat the butter in a large non-stick skillet over medium heat.
Add the shrimp, sprinkle with Old Bay seasoning, and cook 2–3 minutes per side until pink and cooked through; transfer to a plate if they finish before the other steps.
Add the chopped garlic to the skillet and cook 30 seconds until fragrant, then add the baby spinach and cook 3–4 minutes until wilted.
Return the shrimp to the skillet and add the lemon juice, cooked pasta, shredded Parmesan, and chopped parsley.
Toss everything together over low heat until the cheese melts and the pasta is evenly coated; taste and adjust seasoning with salt, pepper, or more lemon juice if desired.
Garnish with lemon slices and extra Parmesan, then serve immediately.
