In a shallow pan over medium heat, melt butter in the olive oil
Finely chop the white parts of the scallions and garlic, then saute them for several minutes in the olive oil/butter mixture.
Add the cayenne and creole seasoning. Saute for about a minute to allow the seasonings to bloom.
Add the crawfish and their fat, and cook for several more minutes.
Add the heavy cream and cook over medium-low heat until the sauce reduces and thickens, 5-10 minutes.
Meanwhile, cook the pasta according to package instructions. Drain.
Add the sauce and crawfish to the pasta, along with the chopped parsely and green onion tops, mix well and serve.