In a large pot, melt the butter over medium heat. Whisk in the flour, stirring frequently, until the roux turns golden brown, about 15 minutes. Meanwhile, have students prepare the rest of the vegetables.
Next, add in the bell pepper, celery, onion, okra (if using), and garlic, stirring frequently. Cook until the vegetables are softened, about 5 minutes. Gradually pour in the vegetable broth, whisking until smooth, then add the mushrooms, tomatoes, Cajun or Creole seasoning, thyme, cayenne (if using), bay leaf (if using), and salt. Stir to combine and bring to a simmer.
Cook for another 5-10 minutes, or until the vegetables are tender. Taste and add more seasoning, if necessary, then serve hot over rice.
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