In large serving bowl, toss spinach, mushrooms, pasta and red onion. In medium skillet, cook bacon over medium-high heat until crisp. Remove with slotted spoon. Pour off all but 2 tablespoons drippings. In small bowl, mix cornstarch, sugar, salt and pepper. With wire whisk, stir cornstarch mixture into drippings in skillet until smooth. Stir in mayonnaise until blended. Gradually stir in water and vinegar. Over medium heat, bring mixture to a boil; stir constantly. Boil 1 minute. Pour over spinach mixture. Add bacon; toss to coat well. Serve immediately.
Makes 8-10 servings.