The base is a boxed mix of Zatarain's which could not be easier.
To a large skillet, saute the mushrooms and onions in the olive oil until tender over medium heat. If using raw shrimp, add them now and saute a minute or two until lightly cooked. Also add the sliced sausage.
Add the stewed tomatoes and break up large chunks with your spoon. Add the jambalaya mix and water. Cover and simmer over low heat 25-30 minutes or until the rice is tender.
If you are using cooked shrimp, add them 5 minutes before serving to heat through.