Start by bringing a large pot of salted water to a rolling boil. Add the fettuccine pasta and cook according to package instructions, typically about 8-10 minutes, until al dente. Drain the pasta in a colander and set aside.

In a large skillet, melt the butter along with olive oil over medium heat. Add the diced onion and bell pepper, stirring frequently for about 5 minutes, until softened and fragrant.

Stir in the minced garlic and cook for an additional minute until fragrant but not browned.

Pour in the heavy cream and chicken broth, stirring well to combine. Let it simmer for about 2-3 minutes until slightly thickened.

Sprinkle in the Cajun seasoning, along with salt and pepper to taste. Stir well and bring to a light simmer.

Add the cooked crawfish tails to the skillet, gently folding them into the sauce. Cook for 5-7 minutes until heated through.

Carefully add the drained fettuccine to the skillet, tossing everything together until the pasta is well coated. Cook for another minute on low heat.

Garnish with sliced green onions and serve immediately.