Melt butter in a large non-stick stockpot over medium-high heat. Add mushrooms and onion; cook about 10 minutes, stirring periodically. Let mixture cool slightly.
In food processor or blender, combine mushroom mixture, 1-1/2 cups rice, cheese and 1 cup broth. Pulse mixture until mushrooms are finely chopped but not pureed and mixture is thoroughly combined. Return mixture to stockpot; stir in remaining rice, broth and water. Bring to a boil; cook uncovered over medium heat 5 minutes.
Heat bacon slices between paper towels in microwave according to package directions. Chop bacon and set aside. Add salt and pepper to soup taste. To serve, ladle soup into bowls; sprinkle with bacon.