In a large Dutch oven over medium-high heat, melt ¼ cup butter. Add flour, and whisk vigorously until combined. Cook, stirring frequently, until a light brown roux forms, about 10 minutes. Remove from heat, and set aside.
In a large skillet, melt ¼ cup butter over medium high heat. Add onion, celery, and pepper, and cook until tender, about 3 minutes. Stir mixture into the roux.
Stir tomatoes, 1 teaspoon salt, and seafood stock into roux, and simmer until thickened, about 15 minutes. Stir in crab boil and filé powder.
In a large skillet over medium high heat, melt remaining 2 tablespoons butter. Add garlic, remaining 1 teaspoon salt, and cook until fragrant, about 30 seconds. Add shrimp, and cook until pink, about 2 minutes per side; add oysters, and cook about 2 minutes. Add white wine, and simmer 2 minutes. Stir seafood mixture into vegetable mixture, and serve immediately. Garnish with chopped fresh parsley, if desired.
Recipe by Southern Cast Iron at https://southerncastiron.com/seafood-gumbo/