In a large pot or Dutch oven, heat the cooking oil over medium-high heat.

Gradually add the flour to the hot oil, stirring constantly to form a roux.

Continue cooking the roux, stirring continuously, until it reaches a rich, dark brown color.

Add the chopped onion to the roux and cook, stirring occasionally, until softened and golden brown.

Slowly pour in the water, stirring to prevent lumps from forming.

Add salt, pepper, and green onions to the pot.

Bring the mixture to a boil.

Add the peeled shrimp and crab (fresh or frozen) to the boiling mixture.

Cook until the shrimp are pink and cooked through, and the crab is heated through.

At this point, the gumbo can be cooled and frozen for later use, if desired.

When ready to serve, reheat the gumbo to a boil.

Add the oysters to the boiling gumbo.

Cook for 1 minute, or longer if preferred, until the oysters are plump and slightly cooked.

Remove the pot from the heat.

Stir in file powder to taste (approximately 2 tablespoons).

Garnish with fresh parsley.

Add 1-2 drops of liquid crab boil for a richer seafood flavor.