Make a medium brown roux with the oil and flour. Add the butter, allowing it time to melt and mix in.
Add the white onions and sauté until they barely brown around the edges. Add the green onions, bell pepper, and garlic, and cook until tender.
Add tomato and two cups of water. Bring to a boil and add celery, parsley, basil and bay leaves. Simmer for 10 minutes.
Add crawfish tails, crawfish fat, salt pepper, and Tabasco. Simmer for 10-12 minutes more. Remember as you add the salt and pepper that the boiled crawfish already have a good bit of both. Serve the etouffee over rice, and topped with finely chopped green onions for garnish.