Step 1

Cook the pasta. Bring a large pot of salted water to a boil. Add the fettuccine and cook until al dente, about 12 minutes. Reserve 1/2 cup of the pasta cooking water, then drain the pasta, but do not rinse.

Step 2

Cook the shrimp. Pat the shrimp dry with paper towels and season with the salt and pepper. Melt the 1 tablespoon butter in a large skillet over medium-high heat, then swirl the pan to coat the bottom with the butter. Add the shrimp in an even layer and cook, until pink and opaque throughout, 1 to 2 minutes per side. Transfer the shrimp to a plate, but do not clean the skillet.

Step 3

Make the Alfredo sauce. Melt the 4 tablespoons butter in the skillet over medium heat. Add the garlic and cook until fragrant, 30 seconds to 1 minute. Whisk in the heavy cream or half-and-half until combined and simmering, 3 to 4 minutes. Whisk in the Parmesan cheese, salt, nutmeg, pepper, and 2 tablespoons of the reserved pasta water. Cook, whisking occasionally, until the sauce thickens to coat the back of a spoon, 3 to 4 minutes.

Step 4

Toss the drained pasta in sauce and top with shrimp. Add the drained pasta to the sauce and toss to coat. Return the shrimp to the pan and toss to combine. Serve garnished with parsley.