Bring a large pot of lightly salted water to a boil. Add the fettuccine and cook 8 to 10 minutes until al dente; drain and set aside.
While the pasta cooks, melt the butter in a large nonstick skillet over medium-high heat.
Add the shrimp to the skillet, sprinkle with Old Bay seasoning, and cook 2 to 3 minutes per side until pink and opaque; remove any overcooked shrimp from heat.
Add the chopped garlic to the skillet and cook 30 seconds to 1 minute until fragrant.
Add the baby spinach and cook 3 to 4 minutes, stirring, until wilted.
Return the cooked pasta to the skillet (or add everything to the pot), pour in the lemon juice, then add the shredded Parmesan and chopped parsley. Toss gently to combine and warm through.
Taste and adjust seasoning with salt, pepper, or more lemon juice if desired. Garnish with lemon slices and additional Parmesan, then serve immediately.