1.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. I use a large fork with the flat side to the bottom of the pan in a side to side motion. 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.
2.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.
3.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.