In a large heavy skillet, melt the butter and heat it to a bubble. Add flour gradually and stir constantly over medium-high heat to make a roux. Cook until light brown--about five minutes.
Lower heat and add green onions, bell pepper, and celery. Saute until limp, but not browned--about three minutes. Keep stirring to avoid burning the roux.
Add wine and bring it to a boil. After a minute, add stock, thyme, bay leaf, and garlic, then return to a boil. Simmer about five minutes more, or until thickened.
Add crawfish tails, tomatoes, parsley, green onions, Creole seasoning, and salt. Simmer for about eight to ten minutes, until sauce can be picked up with a fork. Adjust seasonings with salt and pepper.
Sprinkle a thin but complete layer of cheese over bread. Put the bread into a hot broiler or toaster oven until the cheese melts. Take the bread out and spoon the crawfish sauce down the center of the bread, and top with a sprinkling of Parmesan cheese. Return to the oven until the sauce begins to bubble. Slice into appropriate serving portions and serve hot.