Melt 3 tablespoons of butter in a large saucepan over medium heat.
Add the chopped onion and cook until soft, about 5 minutes.
Sprinkle the flour over the butter and onion mixture.
Stir constantly for 2-3 minutes to cook the flour.
Gradually add the clam juice, stirring continuously to avoid lumps.
Pour in the heavy cream, stirring until the sauce begins to thicken.
Stir in the cayenne pepper, salt, and white pepper.
Then, add the shrimp, scallops, and crawfish. Cook until the seafood is just done.
Complete the sauce by stirring in the remaining butter.
Let it simmer for a minute before serving. Enjoy your sauce over your favorite dish!