Make a roux by heating oil, adding flour, and stirring constantly over medium heat until the mixture turns the color of dark peanut butter or light chocolate; be careful not to burn!
Add the chopped onion, bell pepper, celery and garlic and stir to coat. Cook over low-medium heat for about 10 minutes until vegetables start to soften.
Add RO-TEL and whole tomatoes including all liquids in the cans. Chop the tomatoes either before adding, or by squeezing by hand or with the back of a spoon while in the pot. Cook over medium-low heat for about 30 minutes, until oil starts to bubble to surface.
Add the fish stock, salt, and Cayenne and cook, stirring occasionally, for about an hour, adding stock or water to keep the mixture slightly soupy.
Chop the fish into 2" pieces; place on top of the tomato mixture in the pot, without stirring through, and cover and cook for about 10 minutes until the fish flakes easily with a fork.
Add the green onions and parsley and squeeze the lemon juice in. Stir through, adjust seasonings, and serve over steamed rice in soup bowls.