Bring a large pot of water to boil, with 2 teaspoons salt.
Add the Fettucine Noodles and cook until Al Dente per the package instructions (about 8-9 Minutes). Drain and set aside. Reserve 1 cup of the pasta water in case you need it later.
In a large ovenproof skillet - a cast iron skillet if you have one - heat the olive oil over a medium to medium high heat. If you are using raw shrimp add them now for a few minutes either side, then remove to the side.
Add the chopped onions, garlic and parsley and cook a few minutes until the onions are soft and the garlic is fragrant.
Add the butter, cream cheese, cream, red pepper flakes and salt and pepper. Continue stirring until all has melted.
Stir in the parmesan until melted.
Add the noodles back, along with the shrimp and stir to combine.
Top with the grated mozzarella and place under a broiler for a few minutes until the cheese on top has melted and turned a golden brown.
Notes
Feel free to change up the amount of butter - I usually play it by ear mostly but add anywhere from 6-8 tablespoon butter.Sprinkle a few tablespoons parsley for garnish if you want to pretty it up.Have a few tablespoons parmesan handy to serve with, in case anyone wants extra to sprinkle on top.When reheating this dish you may need a splash of milk to thin it a little.For a little extra texture try sprinkling some bread crumbs on top with the mozzarella.