In 10-inch nonstick skillet, cook sausage and onion over medium-high heat 8 to 10 minutes, stirring frequently, until sausage is thoroughly cooked. Add mushrooms. Cook 4 minutes or until mushrooms are tender. Stir in cooked rice, spinach and sage; cook about 3 minutes or until spinach is wilted and mixture is hot. Divide sausage-rice mixture between squash halves, pressing down on filling so it forms a slight mound over cavity.