In a large heavy pan heat oil over medium-high heat. Add flour to the oil and stir constantly until the flour mixture is slightly lighter than the color of a tarnished penny. This is your roux. Now add the yellow onions and saut? until the onions are clear. Add all the rest of the ingredients except the fish and green onions. (Remove the seeds from the lemon before adding it). Stir well and lower heat to medium. Simmer for 1/2 hour. Salt and pepper the fish and add it and the green onions to the gravy. Simmer until fish is tender. Fillets or whole fish can be used.