Combine the flour and oil in a large heavy pot or Dutch oven over medium heat.
Stirring slowly and constantly, make a roux the color of peanut butter.
Add the onions, bell peppers, celery, and garlic.
Cook, stirring until the vegetables are soft, about 5 minutes.
Add the whole tomatoes and Ro-Tel and stir to blend. Reduce the heat to medium-low and cook, stirring occasionally, until the oil forms a thin layer, like paper, over the top of the mixture, about 30 minutes.
Add the fish stock or water, salt, and cayenne and cook, stirring occasionally, for 1 hour.
The mixture should be slightly thick. (If the mixture becomes too thick, add more stock or water.)
Add the fish, and cover and cook (do not stir) until the fish flakes easily with a fork, 10 to 15 minutes.
Adjust seasoning if necessary, with salt and cayenne.
Add the green onions and parsley.
Serve immediately in soup bowls with steamed rice and pass plenty of hot French bread around the table.