Melt the butter in a large, heavy skillet set over medium heat. Mix in the flour creating a roux. Stir and cook the roux until light brown in colour, about 5 minutes. Add the onion, bell pepper, celery, garlic and Cajun spice and cook, stirring often, for 5 minutes. Mix in the tomatoes and stock or broth.
Bring the mixture to a boil, and then reduce heat until sauce is gently simmering. Simmer etouffee sauce, stirring occasionally, 10 minutes. Mix in the prawns and cook 3 to 4 minutes, or until cooked through. Mix in the parsley. Divide etouffee among serving dishes, sprinkle with green onion, and enjoy.
Note: Cajun spice is sold in bottles in the herb and spice aisle.