Cook the fettuccini according to package directions until al dente; drain and set aside.
In a large skillet over medium-high heat, melt the butter.
Add the minced onion to the skillet and cook 3–4 minutes until softened.
Add the minced garlic and lemon zest and cook 1 minute until fragrant.
Slowly stir in the heavy cream, salt, and pepper until combined.
Whisk in the cubed cream cheese and stir until fully melted and the sauce is smooth; remove from heat and let cool slightly.
Stir in the lemon juice, then add the cooked pasta and minced parsley to the sauce and toss to coat evenly.
Serve immediately topped with grated Parmesan cheese and additional parsley or lemon peel if desired.