Heat the oil in a large stainless-steel or enamel-coated pot over medium heat. Stir in toasted flour, blending well. Stir in green onions. (The flour will stick to the vegetables.) Continue cooking, stirring constantly, for 1 minute or until onions wilt. Add the celery, okra, tomatoes, and heated stock, stirring until well blended. Bring to a boil; reduce heat, and simmer, uncovered, for 40 minutes.
Stir in shrimp, oysters, and crabmeat. Cook 3 minutes or until shrimp turns pink and oysters begin to curl. Stir in the garlic. Let stand 5 minutes. Season with salt and pepper.
*To make toasted flour, spread 3/4 cup all-purpose flour in a shallow baking dish. Bake at 375° for 15 to 20 minutes, stirring often, or until flour is light beige (it will darken when mixed with fat). Cool; process in a blender or food processor to remove any lumps. Store, refrigerated, in an airtight container until ready to use.