Mushroom and onion tart

Ingredients

• 2 red onions, sliced
• 2 tablespoon butter
• 2 tablespoon olive oil
• few sprigs fresh thyme
• 1 teaspoon brown sugar
• 2 tablespoon balsamic vinegar
• 4 - 6 brown mushrooms, stalks removed
• Dough base
• 250 gram plain flour
• 1 teaspoon baking powder
• 1 teaspoon sea salt
• 50 gram grated Parmesan
• 100 millilitre milk
• 50 gram butter, melted
• 1 egg, lightly beaten
• 75 gram blue cheese, to serve