Heat oil in a Dutch oven over medium-high heat. Add onion, carrot, and celery; sauté 6 minutes or until tender. Reduce heat to medium; add mushrooms and garlic. Cook 7 minutes or until mushrooms are tender, stirring frequently. Add water, sage, salt, pepper, and broth; bring to a simmer. Cover and simmer 25 minutes. Remove from heat; cool 5 minutes.
Place half of mushroom mixture in a blender; process until smooth. Pour pureed soup into a large bowl. Repeat procedure with remaining mushroom mixture. Return soup to pan. Stir in half-and-half and sherry; cook over low heat 5 minutes or until thoroughly heated, stirring frequently. Sprinkle each serving with 1 teaspoon parsley.