Bring a large pot of salted water to a boil. Add the fettuccine and cook according to package instructions until al dente, usually about 10-12 minutes. Drain the pasta, reserving about half a cup of the pasta water, and set aside.
Heat the olive oil in a large skillet over medium-high heat. Toss the sliced chicken breast with the Cajun seasoning, salt, and pepper. Add the chicken to the hot skillet and cook until browned and cooked through, about 5-7 minutes. Remove the chicken from the skillet and set aside.
In the same skillet, add the chopped onion and bell pepper. Sauté until softened, about 4-5 minutes. Add the minced garlic and cook for another minute until fragrant.
Reduce the heat to medium-low. Pour in the chicken broth and heavy cream, stirring to combine. Let the sauce simmer gently for 3-4 minutes, allowing it to thicken slightly.
Add the cooked chicken back into the skillet. Then stir in the Parmesan cheese, allowing it to melt smoothly into the sauce. If the sauce seems too thick, add a splash of the reserved pasta water to loosen it up. Taste and adjust seasoning with salt and pepper.
Add the drained fettuccine to the skillet and toss everything together until the pasta is fully coated in the creamy Cajun sauce.
Sprinkle chopped fresh parsley on top for a burst of color and freshness. Serve immediately and enjoy the perfect balance of spicy, creamy goodness.