Cajun Crawfish and Shrimp Étouffée

1/3 cup vegetable oil
1/4 cup all-purpose flour
1 small green bell pepper, diced
1 medium onion, chopped
2 cloves garlic, minced
2 stalks celery, diced
2 fresh tomatoes, chopped
2 tablespoons Louisiana-style hot sauce
1/3 teaspoon ground cayenne pepper (optional)
2 tablespoons seafood seasoning
1/2 teaspoon ground black pepper
1 cup fish stock
1 pound crawfish tails
1 pound medium shrimp - peeled and deveined

Heat the oil in a heavy skillet over medium heat.
Gradually stir in flour, and stir constantly until the mixture turns brown, 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.
It is very important to stir this constantly for a good roux.
If by chance the roux burns, discard and start over.
Add the onions, garlic, celery and bell pepper to the skillet, and sauté 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, and add the crawfish and shrimp.
Cook for about 10 minutes, or until the shrimp are opaque.