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.
While the pasta cooks, melt the butter in a large non-stick skillet over medium-high heat.
Add the shrimp to the skillet, sprinkle with Old Bay seasoning, and cook 2–3 minutes per side until pink and opaque; transfer shrimp to a plate.
Reduce heat to medium, add the chopped garlic to the skillet and cook 30 seconds until fragrant.
Add the spinach and cook 3–4 minutes, stirring, until wilted.
Return the cooked shrimp to the skillet, then add the lemon juice, drained pasta, shredded Parmesan, and chopped parsley; toss until combined and the cheese melts into a light sauce. Season to taste with salt, pepper, or more lemon juice.
Garnish with lemon slices and additional Parmesan if desired, then serve immediately.