Pat the chicken breasts dry with paper towels. If thick, butterfly or pound to an even ~1/2" thickness. Combine 2 tbs Cajun seasoning, smoked paprika, garlic powder, salt and black pepper in a small bowl. Rub olive oil over the chicken, then coat evenly with the spice mixture on all sides.

Cook the chicken (stovetop)

Heat a large skillet (preferably cast iron) over medium-high heat until hot. Add 1 tbs olive oil if skillet is dry. Place chicken in the pan and cook undisturbed 4-6 minutes until a dark sear forms. Flip and cook 4-5 minutes more until internal temperature reaches 165°F (74°C). Remove to a cutting board and let rest 5 minutes, then slice into 1/2" strips or bite-sized pieces.

Pasta

Bring a large pot of water to a rolling boil. Add 1 tbs salt. Add fettuccine and cook according to package directions until al dente (usually 10-12 minutes). Reserve 1/2 cup of pasta cooking water, then drain the pasta and set aside.

Cajun Alfredo Sauce

Begin the roux

In the same skillet used for the chicken (wipe out excess oil leaving browned bits), melt 3 tbs butter over medium heat. Add minced garlic and cook 30-45 seconds until fragrant. Sprinkle 1 tbs flour over the butter and garlic, whisking continuously for 1 minute to cook the raw flour taste.

Add liquids

Slowly whisk in 1 cup whole milk until smooth, then whisk in 1/2 cup heavy cream. Increase heat slightly and bring to a gentle simmer (do not boil vigorously) while whisking to thicken, about 2-3 minutes.

Finish sauce

Lower heat to medium-low. Whisk in 1 cup grated Parmesan until melted and the sauce is silky. Add 1/2 tsp Cajun seasoning, 1/4 tsp red pepper flakes (if using), and 1/2 tsp freshly ground black pepper. Taste and season with salt as needed. If sauce is too thick, whisk in reserved pasta water a tablespoon at a time until desired consistency (you should have a clingy creamy sauce that coats the pasta).

Assemble & Finish

Add drained fettuccine directly to the skillet with the Alfredo sauce and toss gently to coat, using tongs or two forks. If needed, add up to the reserved 1/2 cup pasta water to loosen the sauce so it evenly coats the pasta.

Fold in sliced blackened chicken and toss gently to combine. Warm through 1-2 minutes so chicken reheats and flavors meld.

Taste and adjust seasoning. Transfer to serving plates or a large shallow bowl. Garnish with chopped fresh parsley and extra grated Parmesan. Serve with an optional lemon wedge for a bright squeeze at the table.