Boil the pasta: Bring a large pot of salted water to a rolling boil. Cook fettuccine until just shy of al dente. Reserve at least 1 cup pasta water, then drain.
Pat shrimp dry and season: Use paper towels to dry the shrimp thoroughly. Toss with 1/2 teaspoon salt, 1/4 teaspoon black pepper, and a small pinch of red pepper flakes if using.
Sear the shrimp: Heat a large skillet over medium-high. Add olive oil and 1 tablespoon butter. When hot and shimmering, add shrimp in a single layer. Cook 1–2 minutes per side until pink and just opaque. Transfer to a plate. Do not overcook.
Build the garlic butter base: Lower heat to medium. Add remaining 2 tablespoons butter to the skillet. Stir in minced garlic and cook 30–45 seconds until fragrant, not browned.
Make the Alfredo sauce: Pour in heavy cream. Simmer gently for 2–3 minutes, stirring, until slightly thickened. Whisk in Parmesan a handful at a time until smooth and silky. If the sauce looks too thick, loosen with splashes of reserved pasta water.
Add lemon and adjust: Stir in lemon juice (and zest if using). Taste and adjust salt and pepper. The sauce should taste rich, savory, and bright.
Combine: Add drained pasta to the skillet and toss to coat. Add a bit of pasta water so the sauce clings to every strand.
Return shrimp to the pan: Slide the shrimp (and any juices) back into the sauce. Toss for 30–60 seconds to warm through. Fold in parsley and any optional greens.
Finish and serve: Add a small knob of butter for extra gloss if you like. Plate with more Parmesan and a squeeze of lemon.
