So just a warning--this is hot and spicy.If you're not sure about the level of heat, add less hot sauce to start--you can always add more, but can't take it away once it's in there.
Heat the oil in a heavy skillet over medium heat. Gradually stir in flour, and stir constantly until the mixture turns 'peanut butter' brown or darker, at least 15 or 20 minutes.
This is your base sauce or 'Roux'. It is very important to stir this constantly. If by chance the roux burns, discard and start over.
Once the roux is browned, add the onions, garlic, celery and bell pepper to the skillet, and saute for about 5 minutes to soften. Stir in the chopped tomatoes and fish stock, and season with the seafood seasoning. Reduce heat to low, and simmer for about 20 minutes, stirring occasionally.
Season the sauce with hot pepper sauce and cayenne pepper (if using), and add the crawfish and shrimp. Cook for about 10 minutes, or until the shrimp are opaque.
Garnish with chopped parsley and green onion.
Instead of crawfish, add diced chicken with the shrimp. Delicious.