01 - Bring a large pot of salted water to a boil. Cook fettuccine according to package directions until al dente. Drain and set aside, reserving 1/2 cup of the pasta cooking water.
02 - Pat chicken breasts thoroughly dry with paper towels. Rub both sides generously with Cajun seasoning, salt, and black pepper, pressing the spices into the meat to adhere.
03 - Heat olive oil in a large skillet over medium-high heat until shimmering. Add chicken and cook for 5–6 minutes per side until deeply blackened and cooked through, reaching an internal temperature of 165°F. Transfer to a plate, cover loosely with foil, and rest for 5 minutes before slicing into thin strips.
04 - Wipe out the skillet if needed, then melt butter over medium heat. Add minced garlic and sauté for 1 minute until fragrant, taking care not to brown it.
05 - Pour in heavy cream while stirring constantly. Bring to a gentle simmer, then add Parmesan cheese, black pepper, and salt. Continue stirring until the cheese melts completely and the sauce thickens, about 3–4 minutes. Add a splash of reserved pasta water if the sauce becomes too thick.
06 - Add the cooked fettuccine to the skillet with the Alfredo sauce. Toss vigorously with tongs until every strand is evenly coated. Remove from heat.
07 - Divide the sauced pasta among four serving plates. Arrange sliced blackened chicken on top. Sprinkle generously with fresh chopped parsley and additional grated Parmesan. Serve immediately while hot.