11. Coat chicken breasts thoroughly with Cajun seasoning, pressing spices into meat.
22. Heat olive oil in large cast-iron skillet over medium-high heat. Sear chicken until golden brown and internal temperature reaches 165°F, about 6-7 minutes per side.
33. Remove chicken, let rest on cutting board. Slice diagonally into thin strips.
44. Boil fettuccine in heavily salted water until al dente, approximately 8-9 minutes. Reserve 1/2 cup pasta water before draining.
55. In same skillet, melt butter. Sauté shallots and garlic until fragrant, about 2 minutes.
66. Pour heavy cream into skillet, whisking continuously. Reduce heat, simmer until sauce thickens slightly, around 4-5 minutes.
77. Fold in Parmesan cheese, creating silky smooth sauce. Season with salt and pepper.
88. Toss pasta in sauce, adding reserved pasta water if needed for consistency.
99. Plate by creating pasta nest, arranging sliced chicken on top. Garnish with parsley and green onions.