Bring a large pot of salted water to a boil. Add the pasta and cook according to package directions until al dente. Reserve 1/2 cup of the pasta water before draining.
Drain the pasta and set aside. Drizzle lightly with olive oil if needed to prevent sticking.
2. Cook the Shrimp:
Pat shrimp dry with paper towels and season lightly with salt and pepper.
In a large skillet over medium-high heat, add the neutral oil.
Add shrimp in a single layer and cook for about 2 minutes per side, or until opaque and lightly golden. Remove shrimp from the skillet and set aside.
3. Make the Alfredo Sauce:
In the same skillet, reduce heat to medium. Add butter and let it melt, then add the finely chopped onion. Cook for 3–4 minutes until translucent and soft.
If using garlic, add it now and sauté for 30 seconds until fragrant.
Pour in the heavy cream, stirring constantly, and bring it to a gentle simmer. Let it cook for 2–3 minutes to slightly thicken.
Add Parmesan cheese gradually, stirring until melted and the sauce becomes smooth. Season with salt and pepper to taste.
