Start by cutting the chicken into 4 evenly sized fillets. Coat each piece generously with blackening seasoning, pressing it into the meat to help it stick. Heat a cast-iron skillet over medium-high heat, add olive oil, and sear the chicken for about 4-5 minutes on each side. It should develop a dark, flavorful crust without burning. Remove the chicken from the pan and tent it with foil to keep it warm.
Bring a large pot of salted water to a boil and cook the fettuccine according to package directions until al dente. Reserve ½ cup of the pasta water before draining it. This starchy water can help loosen the sauce later, if needed.
Using the same skillet (don’t wipe it out—that leftover residue adds flavor!), melt butter over medium heat. Add the minced garlic and sauté until fragrant, about 1-2 minutes. Slowly pour in the heavy cream, whisking as you go. Allow it to heat through for 3-4 minutes. Reduce the heat to low and stir in freshly grated Parmesan cheese until the sauce thickens to a creamy consistency.
Add the drained pasta to the skillet and toss it gently with the Alfredo sauce until well-coated. Slice the blackened chicken into strips and arrange it on top of the pasta. Optionally, sprinkle with fresh parsley or red pepper flakes for garnish.