Bring a large pot of salted water to a boil and cook the fettuccine pasta until al dente.
Drain the pasta, reserving half a cup of the pasta water, and set aside.
Melt the unsalted butter in a large skillet over medium heat.
Add minced garlic to the skillet and sauté for about 1 minute until fragrant.
Add the peeled crawfish tails to the skillet and cook for 4-5 minutes until opaque.
Reduce the heat to medium-low and pour in the heavy cream, stirring gently to combine.
Stir in the Cajun seasoning, salt, and freshly ground black pepper.
Allow the sauce to simmer for 5-7 minutes until it thickens slightly.
Gradually add the freshly grated Parmesan cheese, stirring until melted and smooth.
Add the cooked fettuccine to the skillet and toss to coat the pasta with the sauce.
If needed, add a splash of the reserved pasta water to adjust the sauce consistency.
Remove from heat and garnish with freshly chopped parsley.
Serve the crawfish fettuccine immediately while it's hot.