Bring a large pot of salted water to a boil and cook fettuccine according to package instructions until al dente. Reserve 1/2 cup of pasta water before draining.
In a medium bowl, toss the shrimp with 1 tablespoon of Cajun seasoning until evenly coated.
Heat olive oil in a large skillet over medium-high heat. Add the shrimp and sear for 2 minutes per side until pink and opaque. Remove shrimp from skillet and set aside.
In the same skillet, melt the butter. Add the sliced bell peppers and onions, sautéing for 5 minutes until softened.
Stir in the minced garlic and the remaining 2 tablespoons of Cajun seasoning, cooking for 1 minute until fragrant.
Reduce heat to medium and pour in the heavy cream. Bring to a gentle simmer and cook for 3-5 minutes until the sauce begins to thicken.
Whisk in the Parmesan cheese until completely melted and the sauce is smooth. Adjust seasoning with salt and pepper as needed.
Add the cooked pasta to the skillet and toss to coat thoroughly, adding reserved pasta water in small increments if the sauce is too thick.
To serve, mound the pasta high in the center of the plates to create a volcano shape. Top with the sautéed shrimp and garnish with chopped parsley.