Start by bringing a large pot of salted water to a boil. Cook the fettuccine until it's just tender (al dente). Set aside ½ cup of the pasta water, then drain the pasta and keep it aside.
Meanwhile, pat the salmon dry using paper towels. Generously season both sides with Cajun seasoning, salt, and black pepper.
Heat some olive oil in a skillet over medium heat. Sear the salmon fillets for about 3–4 minutes per side, or until fully cooked. Remove them from the skillet and set them aside.
Lower the heat to medium-low. In the same skillet, melt the butter and sauté the minced garlic for about a minute, just until fragrant.
Pour in the heavy cream and add the cream cheese. Stir gently until the cream cheese is fully melted and the mixture turns into a smooth sauce.
Next, add the grated parmesan along with a bit of the reserved pasta water at a time. Stir continuously until the sauce thickens to your liking.
Mix in the lemon juice and taste the sauce. Add more salt or pepper if needed.
Add the cooked fettuccine to the skillet and toss to coat evenly in the creamy sauce. Let it warm through for 1–2 minutes.
Flake the salmon gently and mix it into the pasta, or you can place the fillets whole on top.
Finish with chopped parsley, and if you like, sprinkle on some lemon zest or extra parmesan before serving. Enjoy it warm!