Cook the fettuccine pasta according to package instructions until al dente. Drain and set aside, reserving 1/2 cup of pasta water.
In a large skillet, heat the olive oil over medium heat. Add the sliced smoked sausage and cook until browned, about 5-7 minutes.
Add the diced onion to the skillet and sauté for 3-4 minutes until softened. Stir in the minced garlic and cook for an additional 1 minute until fragrant.
Pour in the heavy cream and bring to a simmer. Stir in the Parmesan cheese and Cajun seasoning, mixing until the cheese is melted and the sauce is smooth.
Add the cooked fettuccine to the skillet, tossing to coat the pasta in the sauce. If the sauce is too thick, add reserved pasta water a little at a time until you reach the desired consistency.
Season with salt and pepper to taste. Remove from heat and garnish with chopped parsley before serving.