Aunt Annie's Shrimp Gumbo

3/4 cup of corn oil
3/4 cup of all purpose flour
3 lbs of cleaned shrimp
1 large yellow onion chopped
1 bell pepper chopped
3 garlic cloves minced
1 celery rib diced
1 bunch green onions chopped
File (ground sassafras leaves)
2 quarts of water
salt and pepper

Make a very dark roux with the oil and flour. (see Basic Roux recipe in Roux, Simple Cajun Recipes Cookbook); add onions, bell pepper, garlic, celery and 1/2 green onions. Saute until clear.
Spoon this mixture into a gumbo pot and slowly add 2 quarts of water stirring to mix thoroughly ; add 2 Tablespoons of File and simmer about 1/2 hour.  Add more water if gumbo is too thick. 
Add salt and pepper to your taste, remembering that when you add the shrimp 15 minutes before serving, that they will absorb the seasoning.
Ladle gumbo over steamed white rice in a large soup bowl.  Add onion tops and dust with File.