In a large pot of boiling salted water, cook the fettuccine according to package instructions until al dente. Drain and set aside, reserving about 1/2 cup of the pasta water.
In a bowl, toss the shrimp with Cajun seasoning until evenly coated.
In a large skillet over medium-high heat, melt 2 tablespoons of butter. Add the seasoned shrimp and cook for about 2-3 minutes on each side, or until they turn pink and opaque. Remove the shrimp from the skillet and set aside.
In the same skillet, add the remaining 2 tablespoons of butter. Once melted, add minced garlic and sauté for about 1 minute until fragrant.
Reduce the heat to medium-low, then pour in the heavy cream, stirring constantly. Gradually add the grated Parmesan cheese, stirring until melted and creamy. If the sauce is too thick, add a splash of reserved pasta water to reach your desired consistency.
Add the cooked fettuccine and sautéed shrimp to the sauce, tossing to coat everything evenly. Season with black pepper and adjust salt if needed.
Remove from heat and let it sit for a minute. Garnish with sliced green onions and parsley before serving.