In a large, heavy saucepan or dutch oven melt butter and whisk in flour until smooth. Cook over medium heat, stirring constantly, until roux is a peanut butter color, about 20 minutes.
Immediately add onions, celery, bell pepper, garlic, and bay leaves and cook until vegetables are very soft, about 8 minutes. Add stock and whisk to combine. Bring to a boil, skim surface, and reduce heat to a simmer. Add crabs and cook for 1 1/2 hours, until thickened and flavorful. If gumbo seems too thick, thin with water or seafood stock.
Add shrimp and fish and cook for 5 to 7 minutes, or until shrimp turn pink and fish is cooked through. Add oysters (with their liquor), parsley, and green onions, and cook until edges of oysters curl, about 5 to7 minutes. Taste and adjust seasoning, if necessary, and serve in large bowls over hot white rice.
Serving suggestion: cooked white rice