Shrimp Alfredo Fettuccine

In a 6-qt. stockpot, cook fettuccine according to package directions. Drain; return to pot., Meanwhile, in a large skillet, heat butter over medium heat. Add onions; cook and stir 8-10 minutes or until tender. Add garlic; cook 1 minute longer. Stir in flour, salt and pepper until blended; gradually stir in cream. Bring to a boil, stirring constantly; cook and stir 2-4 minutes or until thickened., Stir in shrimp, lemon zest and lemon juice; cook 3-4 minutes or until shrimp turn pink. Stir in cheese and parsley. Add to fettuccine; toss to combine.