In a large pot of salted boiling water, cook the fettuccine according to package instructions until al dente. Drain and set aside, reserving about 1 cup of pasta water.
In a large skillet, heat olive oil over medium heat. Add the shrimp and season them with salt, pepper, and 1 teaspoon of Cajun seasoning. Cook until shrimp are pink and opaque, about 3-4 minutes. Remove from skillet and set aside.
In the same skillet, add the chopped onion and cook until translucent, about 3 minutes. Then add the minced garlic, red bell pepper, and green bell pepper. Sauté until peppers are softened, about 5 minutes.
Lower the heat to medium-low, and pour in the heavy cream to the skillet. Stir well and allow it to simmer for about 2-3 minutes. Gradually add the Parmesan cheese, stirring until melted and smooth.
Stir in the cooked shrimp and the remaining Cajun seasoning. If the sauce is too thick, add reserved pasta water a little at a time until desired consistency is reached.
Add the fettuccine to the skillet, tossing everything together until the pasta is well coated with the creamy sauce. Adjust seasoning with more salt and pepper if needed.
Divide the pasta among plates, garnishing with fresh parsley for a pop of color.
Notes
Adjust the Cajun seasoning to taste for more or less spice.