In a large 12 quart pot on medium-high heat, heat oil until hot then add flour and stirring constantly brown flour to form the roux. The roux is done when it is the color of a tarnished penny. Add onions, bell pepper and green onions and stirring very often cook until onons are clear, about 10-15 minutes. Add water, salt, pepper, and Cajun seasoning and cook stirring occasionally for 15 minutes. Add crabs and lower heat to medium and cook for 1 hour. Take the claws off the crabs to cook them so stirring will be easier. If you want you can also remove and discard the legs but I leave them on. Skim grease from top of stew then add shrimp, stir well and cook for another half hour. Skim grease if there is any and serve over rice.