HEAT THE OIL IN A SKILLET and add the mushrooms, garlic, onion, vinegar, water, salt and pepper. Cover and cook 3 minutes. Remove the cover and cook until mushrooms are tender, another 5 minutes. Remove from the heat, transfer the mushrooms to a bowl and add the parsley. Toast or grill the bread. Preheat a broiler. Place the bread on a baking sheet and top with mushrooms. Lay a piece of cheese on top and place under the broiler until melted. Serve immediately.