1. Butterfly shrimp by making a deep slit down back of each from large end to tail, cutting to but not through inside curve of shrimp. Devein shrimp.
2. Cook onions in hot oil in a large Dutch oven over medium heat, stirring often, 35 to 40 minutes or until golden brown. Add tomato sauce, and cook, stirring occasionally, 25 to 30 minutes or until thickened. Add diced tomatoes with green chiles, celery, bell peppers, and 2 cups water.
3. Bring to a boil over medium-high heat; reduce heat to medium, and simmer 30 minutes or until sauce thickens slightly. Add 3 garlic cloves, and simmer 30 minutes, stirring in 1 to 2 cups of water as needed to maintain a stew-like consistency. Stir in sausage, and simmer 30 minutes, stirring in 1 to 2 cups water as needed. Skim grease from surface as needed.
4. Meanwhile, prepare rice according to package directions.
5. Stir green onions, parsley, Cajun seasoning, and remaining 3 garlic cloves into sausage mixture. Simmer 10 minutes. Add shrimp, and bring to a boil over medium-high heat.
6. Stir together 1/4 cup cornstarch and 1/2 cup water. Stir into shrimp-and-sausage mixture, stirring just until mixture is thick and glossy. Serve with hot cooked rice.