Bring a large pot of water to a boil, add salt (to taste), and cook 1 lb fettuccine according to package instructions until al dente. Before draining, reserve a small amount of the pasta cooking water if you think you may need it to thin the sauce. Drain the pasta and set aside.

Grate the 4 oz freshly grated Parmesan (if not already grated) and set it aside to come to room temperature.

Make the Alfredo sauce: heat a large saucepan over medium heat. Add 1/2 cup butter and let it melt. Add the 4 cloves garlic (finely minced) and cook, stirring, for about 1 minute until fragrant.

Pour in 2 cups heavy cream and whisk to combine. Bring the mixture to a gentle simmer and cook for 3–4 minutes, stirring often.

Reduce the heat to low and add the grated Parmesan a little at a time, whisking until the sauce is smooth and the cheese is fully melted. Season the sauce with salt and pepper to taste. Keep the sauce on the lowest heat while you cook the shrimp.

Cook the shrimp: pat 1.5 lbs large shrimp dry and season with salt and pepper. Heat 1 Tablespoon oil (or butter) in a large skillet over medium-high heat. Add the shrimp in a single layer and cook, stirring or flipping once, until pink and opaque, about 2–3 minutes total. If your skillet is crowded, cook the shrimp in batches.

Add the drained pasta to the Alfredo sauce and toss to coat. If the sauce is too thick, loosen it with a splash of the reserved pasta cooking water until you reach the desired consistency.

Serve the pasta topped with the cooked shrimp. Garnish with fresh chopped parsley, if desired.