1. Saute onion and garlic in 3 tbsp water; stirring often, until softened, about 5 minutes. Add mushrooms and cook until they begin to release their juices, about 10 minutes.
2. Add broth, wine, soy sauce, thyme and sage and simmer for 10 minutes.
3. Mix cornstarch and water in a small bowl. Stir into the sauce and simmer, stirring often, until slightly thickened, about 10 minutes more. Season with pepper.