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.
Prep Time 30 Min Cook Time 30 Min Ready In 1 Hr.