Preheat oven to 350 degrees. Lightly grease a 2-quart casserole.
To prepare the rice, bring 1 1/4 ounces of water to a boil in a small saucepan. Add the rice, cover, reduce heat, and simmer for 35-40 minutes or until the water is just absorbed. Remove from the heat and allow to sit, covered for 5 minutes. Fluff with a fork. (At this point, you may cool and then cover and refrigerate the rice for up to 2 days.)
Heat a large skillet over medium heat and cook the sausage until evenly brown, crumbling the meat as it cooks. Remove to a mixing bowl.
In the same skillet over medium heat, cook the mushrooms, onion, and celery until the mushrooms release their liquid and the vegetables are beginning to brown, about 8-10 minutes. Add the garlic for the last 30-60 seconds, stirring until you can smell it but it is not brown. (If the skillet seems dry at any point, add a drizzle of olive oil.) Remove from the heat, and allow to cool for a few minutes.
To the sausage, add the cooked rice, mushroom mixture, parsley, egg, water chestnuts, Parmesan, thyme, salt and pepper.
Mix until combined and then turn into the prepared baking dish.
Bake for 15-20 minutes or until hot throughout. Garnish with additional parsley, if desired.