Slice mushrooms. In a 10-inch heavy non-stick skillet cook garlic in butter over moderately low heat, stirring, until pale golden. Add onion and cook, stirring, until softened. Add mushrooms and soy sauce and sauté mixture over moderately high heat, stirring, until liquid mushrooms give off is evaporated and mushrooms begin to brown.
Add wine and vinegar and boil until liquid is evaporated. Stir cornstarch mixture and add to skillet with sugar. Bring mixture to a boil, stirring, and simmer 2 minutes. Stir in parsley and season gravy with salt and pepper. Gravy may be made 1 day ahead and chilled, covered. When reheating gravy, add water if necessary to thin to desired consistency.
Serve gravy spooned over baked or mashed potatoes and/or steak or chicken.