In a heavy pot, mix flour and vegetable oil over medium heat.
Stir constantly until the roux turns dark brown.
Add onion, bell pepper, celery, and garlic; cook until soft.
Add sausage and chicken; brown lightly.
Slowly stir in chicken stock until smooth. Add bay leaves, thyme, cayenne, salt, and pepper.
Bring to a boil, then reduce to a simmer for 30–40 minutes.
Add shrimp and cook until pink, about 3–5 minutes. Stir in file powder if using.
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