Bring a large pot of lightly salted water to boil. Add pasta, and cook until al dente, about 8 to 10 minutes. Drain, and set aside.
Meanwhile, heat 1/4 cup butter in a large skillet over medium heat. Stir in mushrooms, and cook until soft. Remove mushrooms, and set aside. Wipe out skillet.
Heat 1/2 cup butter in the skillet over medium-high heat. Stir in onions, and cook until soft and translucent. Stir in garlic, and cook 1 minute. Add cream cheese, and cook until melted.
Reduce heat to medium low, and stir in flour, whipping cream, and 1/4 cup Parmesan cheese. Stir in shrimp, and season with Cajun seasoning, salt, and pepper. Cook until shrimp are pink and sauce thickens, about 3 minutes. Remove from heat. Fold mushrooms, green onions, and fettuccine into sauce. Sprinkle with remaining 1/4 cup Parmesan, and serve.
