01 - Bring a large pot of salted water to a boil. Add fettuccine and cook according to package directions until al dente. Drain well, reserving 1/2 cup of pasta water.
02 - Pat chicken breasts dry and season both sides evenly with Cajun seasoning. Heat olive oil in a large skillet over medium-high heat. Cook chicken breasts for 5 to 6 minutes per side until golden and internal temperature reaches 165°F. Transfer to a plate and allow to rest for 5 minutes, then slice thinly.
03 - In the same skillet, melt unsalted butter over medium heat. Add minced garlic and sauté until aromatic, about 1 minute.
04 - Stir in heavy cream and bring to a gentle simmer. Whisk in freshly grated Parmesan, salt, black pepper, and red pepper flakes. Continue whisking until cheese is fully melted and sauce is smooth, about 2 to 3 minutes.
05 - Add fresh baby spinach to the Alfredo sauce and cook just until wilted, approximately 1 minute.
06 - Return cooked fettuccine to the skillet, tossing thoroughly to coat each strand in sauce. If needed, add reserved pasta water incrementally to achieve a creamy consistency.
07 - Arrange sliced Cajun chicken on top of the pasta. Toss gently to combine, or serve chicken over individually plated pasta portions.
08 - Finish with chopped fresh parsley and extra grated Parmesan. Serve immediately.