Toss diced chicken breast with Cajun seasoning until well-coated.
Heat olive oil in a large skillet over medium-high heat until shimmering.
Add chicken in a single layer and sear for 4-5 minutes until golden brown on all sides but not fully cooked. Remove chicken and set aside.
Lower heat to medium and add minced garlic to the skillet. Sauté gently for about 1 minute until fragrant.
Pour in chicken broth and heavy cream, stirring to combine. Bring mixture to a gentle simmer.
Add uncooked fettuccine in a single layer, pressing it down gently so it’s submerged in the liquid.
Simmer uncovered, stirring occasionally to prevent sticking, until pasta is al dente and liquid has reduced into a creamy sauce, about 12-15 minutes. Add more broth or cream if sauce thickens too much before pasta is done.
Stir the seared chicken back into the pot. Sprinkle in grated Parmesan cheese and stir until melted and sauce is creamy.
Taste and adjust seasoning with salt and pepper as needed.
Remove from heat and let rest for a couple minutes to thicken slightly.
Sprinkle chopped parsley over the top and serve immediately.