Heat the butter in a large pot over medium-high heat until melted, then stir in the flour. Cook, stirring constantly, until the roux turns dark brown, 10 to 15 minutes.
Add the onions, bell pepper, celery, and garlic. Cook the vegetables, stirring, until soft, about 5 minutes.
Stir in the seafood stock, Worcestershire sauce, salt, Cajun seasoning, cayenne pepper, and bay leaf. Bring the mixture to a boil, then reduce to medium-low heat. Simmer for at least 90 minutes. Add more stock or water if necessary.
Add the shrimp, fish, and squid and simmer until just cooked through, about 5 minutes. Remove the bay leaf. Stir in some fresh parsley. Serve the seafood gumbo in bowls with rice and top with a bit more parsley. Enjoy!