Open-face Mushroom-brie Sandwiches

• 130 gr trimmed assorted mushrooms
• 60 gr onion (finely diced)
• 30 ml olive oil
• 16 ml wine vinegar (red)
• 16 ml water
• 3 gr garlic (minced)
• 3 gr parsley (finely chopped)
• 4 slices country bread (sliced)
• 4 slices brie cheese (sliced)