Heat olive oil in skillet and add mushrooms, garlic, onion, vinegar, water, salt and pepper to taste. Cover and cook 3 minutes. Remove cover and cook until mushrooms are tender, another 5 minutes. Remove from heat, transfer mush rooms to bowl and add parsley. Toast or grill bread. Place bread on baking sheet and top with mushrooms. Place piece of cheese on top and place under broiler until melted. Serve immediately. Makes 4 sandwiches.