1. Preheat oven to 350 degrees F. Spray a medium baking dish with vegetable cooking spray.
2. Spray a medium saucepan with vegetable cooking spray. Heat pan over medium heat. Add onion. Cook, stirring frequently until tender, about 5 minutes. Stir in water and soup. Bring mixture to a boil; remove from heat.
3. Stir rice with seasoning packets and 1 cup mozzarella into saucepan. Spoon rice mixture into prepared dish; cover.
4. Bake casserole until rice is tender, about 15 minutes. Top with remaining mozzarella. Bake, uncovered, until cheese melts, about 5 minutes longer.