First, you make a roux. In large, heavy stock pot on medium heat, add 2 cups vegetable oil and bring to temperature. Add flour gradually. It will sizzle when it is at the right temperature. Using a whisk, stir roux constantly. Stir until it reaches a peanut butter color. When it reaches the desired color, add the onions, celery, bell pepper, garlic and 1 cup of green onions (in that order) until all ingredients are cooked down or caramelized.
Next, add water, okra, Ro-Tel tomatoes, seasonings and bring to boil. After it reaches boiling, reduce heat to low and let simmer for at least an hour. Add seafood and continue to cook on medium heat for about 15 to 20 minutes.
Taste for seasoning and texture. Add file to thicken. Stir in the remaining green onions and simmer for a few more minutes. Serve as a soup with a little steamed rice. Makes about 8 to 10 gallons of gumbo. Can cut recipe in half to reduce.Makes 6 to 8 Source: